354 .NET Developer Interview Questions & Answers

Create a class that inherits from Exception, give it a clear name, and keep any extra context the caller needs, such as an id, a code, or the original failure. Then throw that type and catch it where special handling is needed.

This question is asking how to make your own error class for a problem that needs a clearer name and more details than a normal message. You use it when one kind of failure needs special handling or better logging. The class can carry things like an order id, a code, or the original failure so the caller can understand what happened. The point is to make the problem easier to find, easier to log, and easier to handle in the right place.

1. Do you want an error code or record id on the exception?
2. Should callers catch this type directly or wrap it again?

In C#, a custom exception is a class that inherits from Exception. The name should describe the failure clearly, such as OrderNotProcessedException or PaymentFailedException. Add only the details that callers really need, such as an order id, an error code, or another exception that caused the failure.

The main value is context. A plain message is often not enough for logs, support, or recovery logic. A custom type lets callers catch one specific failure and react in a focused way. If you wrap another error, pass it as innerException so the original cause is not lost.

Use a custom exception when the built in exception types do not describe the business problem well enough. Do not use one for every simple validation case. For bad input, ArgumentException or a similar built in type is often better. In production, keep the class small, keep the extra data readable, and preserve the original cause whenever you can.

A common mistake is to put all detail only in the message and leave callers to parse strings. Another mistake is to hide the original cause and lose the inner exception. A third mistake is to create a custom exception for simple validation when ArgumentException is enough. Keep the type focused and give it only the context that callers really need.

Start with the rule: inherit from Exception, use a clear name, add read only context fields, and pass the original error as innerException when you wrap another failure.

A delegate in C# is a type safe object that can reference one or more compatible methods. I can store a method in a delegate variable and invoke it later, which makes delegates useful for callbacks and event patterns. The compiler checks that the method is compatible with the delegate signature. Delegates are immutable, so combining or removing methods produces another delegate value rather than changing the existing delegate object.

A delegate lets a program remember an action and run that action later. Instead of deciding every action in one place, one part of the program can give another part a piece of work to perform. The receiver does not need to know the details of that work. It only needs to know what information must be provided and what result may come back. This makes programs easier to connect and change. It is commonly useful when one part needs to call another part after something happens.

1. Would you like me to explain both single method and multiple method delegates?
2. Should I also compare delegates with events and lambda expressions?

A delegate is a C# reference type that represents callable methods with a compatible parameter list and return type. For example, delegate int Operation(int a, int b); defines a delegate that can reference a compatible method that accepts two int values and returns an int.

A delegate instance contains information about the method to call. For an instance method, it also keeps a reference to the target object. For a static method, no target object is required. Calling the delegate invokes the referenced method.

The compiler provides type safety by checking delegate and method compatibility. Method groups and lambda expressions can be converted to compatible delegate types. Common built in delegate types include Action, Action<T>, Func<TResult>, and forms of Func with input parameters.

Delegates are immutable. Adding a method with += creates a combined delegate value containing an invocation list. Removing a method with -= also produces another delegate value. A multicast delegate normally invokes its methods in invocation list order. If an invocation throws and that exception escapes, later methods are not invoked. For a delegate with a return value, normal multicast invocation returns the value produced by the last method that successfully runs.

Delegates are also the basis of the C# event pattern. An event exposes controlled subscription and removal while normally preventing outside code from directly invoking the event.

In production code, consider lifetime and allocation behavior. A retained delegate can keep its target object reachable. A capturing lambda can also keep captured state reachable and may require compiler generated storage. Creating and combining delegates can allocate objects, so allocation sensitive code should avoid unnecessary delegate creation after measurement shows it matters.

A common mistake is thinking a delegate stores only a method name. A delegate for an instance method also keeps a reference to its target object. Another mistake is assuming any method can be assigned to any delegate even when their signatures are incompatible. Candidates may also confuse delegates with events. A delegate value can normally be invoked by code that holds it, while an event controls how outside code subscribes and removes handlers and restricts outside invocation. Another mistake is assuming every method in a multicast delegate always runs. If one invocation throws and the exception escapes, later methods are not called. It is also incorrect to think += mutates the existing delegate object because delegates are immutable.

Start by saying that a delegate is a type safe callable reference represented by a C# reference type. Then show a small method assignment and invocation. Mention callbacks and events next. Finish with one practical detail such as delegate immutability, multicast exception behavior, or the fact that an instance method delegate can keep its target object reachable.

I use Func when the operation must return a value, Action when it returns nothing, and Predicate when one input is tested and the result is true or false. Func puts its return type last and can have zero through sixteen input parameters. Action can have zero through sixteen input parameters and always returns void. Predicate takes exactly one input and returns bool. They are standard .NET delegate types commonly used with lambdas, method groups, callbacks, collection APIs, and LINQ.

The practical difference is what the piece of work must produce. One form is used when some work receives values and gives a value back. Another is used when work performs an action but gives nothing back. The third is designed for a yes or no test on one value. Choosing the correct form makes the purpose of a callback easier to understand. It also lets library code accept behavior without requiring a new custom type for every small operation. The main decision is therefore whether the callback returns a value, returns nothing, or answers a condition.

1. Would you like an example using lambdas and method groups?
2. Should I also compare Predicate with Func that returns bool?

Func, Action, and Predicate are delegate types provided by .NET. A delegate is a type safe object that can refer to callable code such as a compatible method or lambda.

Func is used when the called code returns a value. Its last generic type argument is the return type. Any earlier generic arguments are input parameter types. Func<int> represents code that takes no input and returns an int. Func<int, int, int> represents code that takes two int values and returns an int. The standard Func family supports zero through sixteen input parameters.

Action is used when the called code returns void. Action represents code with no parameters. Action<string> represents code that accepts one string and returns nothing. The standard Action family supports zero through sixteen input parameters.

Predicate<T> represents code that accepts exactly one T value and returns bool. It is used by APIs that specifically ask whether an item matches a condition. For example, List<T>.Find and List<T>.RemoveAll accept Predicate<T>. LINQ methods commonly use Func<T, bool> instead. Enumerable.Where is a common example.

A compatible lambda or method group can be converted to the delegate type expected by the assignment or method call. However, Predicate<T> and Func<T, bool> remain different delegate types. An existing instance of one is not directly assignable to the other merely because their parameter and return shapes match.

Calling through a delegate has some invocation overhead compared with a direct call. Creating a delegate can also allocate an object. A lambda that captures local state can require a compiler generated closure object so that captured values remain available after the surrounding scope continues. These costs are usually small in ordinary code but can matter in frequently executed paths.

A common mistake is saying that Func always takes one input. Func can have zero through sixteen input parameters, followed by one return type. Another mistake is saying that Action returns a value. Action always returns void. Predicate<T> takes exactly one T input and returns bool. Candidates also sometimes say that Predicate<T> and Func<T, bool> are the same delegate type. They can represent callable code with the same basic parameter and result shape, but they are distinct delegate types. Another mistake is assuming every lambda has the same allocation behavior. A lambda that captures surrounding local state can require a closure object, so unnecessary captures can increase memory use in frequently executed code.

Start with the return rule because it makes the three types easy to separate. Say Func returns a value, Action returns void, and Predicate tests one value and returns bool. Then give one short signature for each. If the interviewer asks for more depth, explain that Predicate<T> and Func<T, bool> are distinct delegate types and mention that captured lambdas can require additional allocations.

In C#, an event is a controlled notification mechanism built on a delegate. A publisher exposes an event, subscribers register handler methods with the event, and the publisher raises the event when something happens. Subscribers normally use += to register and -= to unregister. Outside code can add or remove handlers, but it cannot normally invoke the event or replace its complete subscriber list. The declaring type controls when the notification is raised.

Events let one part of a program tell other parts that something happened without needing to know exactly who is listening. For example, an order can announce that it was completed. One part might then send a message. Another part might update a report. The order does not need to call each interested part directly. Other parts can choose to listen, and they can stop listening when they no longer need the notice. This keeps responsibilities separate and makes the program easier to change or extend.

1. Should I explain both custom delegate events and EventHandler based events?
2. Should I cover subscriber lifetime, exceptions, and memory retention?

A C# event is built on a delegate. The delegate defines the method signature that subscribers must match. A publisher can declare an event such as public event EventHandler<OrderCompletedEventArgs>? OrderCompleted;.

A subscriber registers a compatible handler with publisher.OrderCompleted += HandleOrderCompleted;. A multicast delegate can contain multiple registered handlers. When the publisher raises the event, the handlers in the current invocation list are called in their delegate invocation order. A subscriber removes its handler with publisher.OrderCompleted -= HandleOrderCompleted;.

The event keyword adds an important access restriction. Normal code outside the declaring type can add and remove handlers, but it cannot directly invoke the event or assign a new complete delegate value to it. The declaring type controls when the event is raised.

A common raising pattern is OrderCompleted?.Invoke(this, args). The null conditional operator safely handles the case where no handlers are registered. The receiver is evaluated once for that invocation. With concurrent subscription changes, an already captured invocation can still call a handler that was removed at nearly the same time, so unsubscription is not a guarantee that an overlapping invocation cannot reach that handler.

Normal event invocation is synchronous. A slow handler delays the publisher. If a handler throws an exception and that exception is not handled, invocation stops at that point and later handlers in the invocation list are not called.

Events are useful when one publisher can have zero or many interested listeners. They are less suitable when the publisher needs one specific collaborator or needs a result returned directly. A method call or interface is usually clearer for that case.

Subscriber lifetime also matters. A publisher stores delegate references to registered handlers. An instance handler contains a reference to its target object, so a reachable long lived publisher can keep a shorter lived subscriber reachable. Subscription and removal also create updated delegate values because delegates are immutable. Raising the event in this example allocates a new event data object each time Complete is called. These costs are usually small, but frequent events or large subscriber sets should be designed carefully.

A common mistake is treating an event like an ordinary public delegate field. The event keyword prevents normal outside code from invoking the event or replacing its complete subscriber list. Another mistake is forgetting to remove a shorter lived subscriber from a reachable long lived publisher, which can keep the subscriber reachable through the delegate target reference. Candidates also sometimes assume handlers run asynchronously. Normal event invocation is synchronous. Another mistake is assuming that removing a handler guarantees that no overlapping invocation can still call it. Concurrent invocation may already have captured the handler. Finally, an unhandled exception from one handler can prevent later handlers from running.

Start by saying that an event is a controlled delegate based notification mechanism. Then explain the three important actions: the publisher declares and raises the event, subscribers register with +=, and subscribers remove handlers with -=. Finish with the access restriction, synchronous invocation behavior, exception effect, and subscriber lifetime concern.

Attributes are metadata tags in C# that I place in square brackets. They attach extra facts to a type, member, or parameter, and tools or frameworks can read those facts later. I use them for things like marking code as obsolete, guiding serialization, or helping reflection based code make decisions.

Attributes are small notes I attach to code. They do not change the main logic by themselves. They tell tools, the compiler, or the runtime extra facts about a class, method, property, or parameter. That makes code easier to guide without adding special if statements everywhere. For example, one attribute can warn that a method should not be used, while another can help a serializer rename or skip a property. In an interview, I would say they are a clean way to attach metadata to code.

1. Do you want me to focus on built in attributes or custom attributes?
2. Should I explain how reflection reads attributes at runtime?

In C#, attributes are metadata. You write them in square brackets above a declaration, such as a class, method, property, or parameter. Most attributes do not hold the real business logic. Instead, they store extra information in the assembly so the compiler, the runtime, or another library can read it later.

A simple example is Obsolete. If I mark a method with that attribute, the compiler can warn other developers that the method should not be used. Another common use is serialization. A JSON library can read attributes to decide how to name a property or whether to ignore it. Test frameworks also use attributes to find test methods.

The main value is that the code stays clean while still carrying useful instructions. That is why attributes fit well with frameworks and tools that work by reading metadata. Some attributes affect compile time warnings. Others matter only when a framework or reflection code reads them at runtime.

There are also limits. An attribute is not the same as the behavior itself. If no tool or library reads it, it does nothing useful. Reflection can also add some overhead if code checks attributes often. In production, I use attributes when I want a clear declarative hint. I do not use them for core business rules when plain code is easier to follow.

A common mistake is thinking attributes are the same as program logic. They are not. Another mistake is assuming every attribute changes runtime behavior by itself. Many attributes only matter if a compiler, framework, or reflection code reads them. People also forget that custom attributes must inherit from System.Attribute. Another mistake is using attributes for core business rules when plain code would be clearer.

Start with the simple idea that attributes are metadata in square brackets, then give one built in example like Obsolete or serialization.

Anonymous types let me create a small temporary object without declaring a named class first. C# infers the property names and types, and the compiler creates a reference type with read only properties. I mainly use them for local projections, especially with LINQ. They are usually kept inside a method because the generated type name cannot be written directly in normal C# source code, so it is not suitable as a normal method signature or public contract.

Anonymous types are useful when I need to group a few values for a short local task but do not need to create a permanent named class. I can create an object with the values I need, and C# works out the names and types of its properties. I can then read those values by property name. This is common when a query needs only part of a larger object, such as a customer name and city. The shape is normally kept inside one method because its generated type does not have a name that normal C# source code can directly use.

1. Do you want me to focus on anonymous types in LINQ projections?
2. Should I compare anonymous types with named classes, records, or tuples?

An anonymous type is a compiler generated reference type created from an anonymous object creation expression such as new { Name = "Maya", Age = 30 }. I do not declare a class for this shape. The compiler infers each property name and type from the expressions I provide. A local variable normally uses var so the compiler can preserve the exact generated type.

The generated type is a sealed reference type. Its properties are public and read only. I can read person.Name, but I cannot later assign another value to that property. The compiler generates the constructor and the property accessors needed to hold those initial values.

A common use is a LINQ projection. For example, a query can select only Name and City from a larger customer object. This creates a small shape that is useful for the remaining local query work without requiring a separate named class.

Anonymous types are normally kept local because their generated type names cannot be referenced directly in normal C# source code. That prevents using the exact anonymous type as an ordinary declared method return type, parameter type, field type, or public contract. The object can technically be treated as object or dynamic, but doing that either loses direct static access to its anonymous properties or gives up compile time member checking, so it does not remove the main design limitation.

The compiler also generates value based Equals and GetHashCode behavior. Instances of the same anonymous type compare their corresponding property values. Property names, compile time property types, and property order determine whether anonymous object initializers in the same program produce the same anonymous type.

Because anonymous types are reference types, creating an instance normally allocates an object that is managed by the garbage collector. This cost is usually acceptable for local projections, but large numbers of temporary objects can create allocation pressure. For reusable domain data, shared contracts, method boundaries, or public APIs, I would normally choose an explicit named type instead.

A common mistake is treating an anonymous type as the same thing as dynamic. It is not. With var, the compiler knows the exact generated type and checks property access at compile time. Another mistake is expecting its properties to be writable after creation. They are read only. Candidates also sometimes assume that an anonymous type is a value type, but it is a reference type and creating an instance normally requires object allocation. Another mistake is trying to make the anonymous type itself a normal method return type, parameter type, field type, or public contract even though its generated name cannot be referenced directly in normal C# source code.

Start by saying that an anonymous type is a compiler generated reference type for a temporary data shape. Then mention inferred property names and types, read only properties, var, and LINQ projections. Finish with the key limitation that its generated type name cannot be written directly in normal C# source code, so anonymous types are best kept in local implementation logic.

Reflection lets C# code inspect types, properties, methods, fields, constructors, and attributes while the program is running. I would use APIs such as Type, PropertyInfo, and MethodInfo when the exact type or member must be discovered at runtime. It is powerful for framework and infrastructure code, but lookup and reflective invocation generally cost more than direct strongly typed access, so I avoid repeated reflection on hot execution paths when a direct approach is practical.

Reflection lets a running program examine information about the code it is working with. For example, the program can discover what properties an object has, what actions its type provides, or what extra labels were placed on its code. It can make decisions using information that was not directly written into the normal path of the program. This is useful when building reusable tools that must work with many different types. The tradeoff is that discovering and using this information while the program runs takes more work than calling known code directly.

1. Should I explain both inspecting members and invoking them with reflection?
2. Would you like a production example involving attributes or properties?

Reflection in .NET is the ability to inspect metadata about assemblies, types, and their members at runtime. System.Type is central to reflection. From a Type object, code can discover constructors, methods, properties, fields, events, interfaces, generic information, and attributes. Objects such as PropertyInfo and MethodInfo describe individual members. ([learn.microsoft.com](https://learn.microsoft.com/en-us/dotnet/fundamentals/reflection/overview?utm_source=chatgpt.com))

A program can obtain a Type from a known type with typeof or from an object with GetType. It can then use methods such as GetProperties or GetMethod to discover members. Reflection can also read attributes, get or set property values, create objects, and invoke methods dynamically.

This is useful when reusable code cannot know every application type in advance. Typical examples include serializers, dependency injection infrastructure, plugin discovery, object mapping, test tools, and code that reads custom attributes.

The tradeoff is runtime cost and weaker compile time checking. Direct code already identifies the member being accessed. Reflection adds metadata lookup and runtime validation. Reflective invocation can also require objects such as argument arrays and can cause boxing when value type arguments are represented as object values. A wrong member name, incompatible argument, or missing member may therefore fail only when that path runs.

For production code, I use reflection when runtime discovery provides real value. If the same member metadata is used repeatedly, I can reuse the discovered information rather than repeat the lookup. If invocation is performance sensitive, a cached delegate, generated code, or direct strongly typed code may be more appropriate. Reflection also needs extra care with trimming and Native AOT when required members cannot be determined statically. Long lived caches should also be designed carefully when types come from collectible AssemblyLoadContext instances because retained type or member metadata can prevent those loaded components from being collected.

A common mistake is treating reflection as if it has the same behavior and checking as normal direct member access. Reflection performs runtime discovery, so requested members may not exist and errors can appear only when the code executes. Another mistake is using GetProperty or GetMethod without checking for null. Developers can also forget that visibility, inheritance, overloads, parameter types, and BindingFlags can change which member is found. Repeating the same metadata lookup inside a hot loop can add unnecessary work. Reflective invocation can also add allocations or boxing depending on the arguments and API used. Another production mistake is assuming every reflection pattern is automatically safe with trimming or Native AOT. Finally, caches that permanently retain Type or MemberInfo objects can be a problem when the application expects a collectible AssemblyLoadContext to unload.

Start by saying that reflection inspects type metadata at runtime. Then name Type, PropertyInfo, and MethodInfo as examples. Give one practical use such as reading attributes or discovering properties. Finish with the main tradeoff: reflection provides runtime flexibility, but direct strongly typed code is normally simpler, safer at compile time, and cheaper to execute.

IEnumerable is mainly used when the data is already in the application, while IQueryable is used when a query provider can inspect the query and decide how to execute it. With IEnumerable, LINQ operations normally use delegates and run as the sequence is enumerated. With IQueryable, LINQ operations build an expression tree that a provider can translate. For example, Entity Framework Core can translate supported filtering and projection into a database query. I use IQueryable while I still want the provider to do that work, then materialize the results before application side processing.

The simple difference is where the work happens. IEnumerable is useful when the application already has the data it needs. You can then read that data and apply operations such as filtering or selecting items. IQueryable is useful when another system can understand the requested work before returning the data. This can allow only the needed data to be returned instead of bringing everything into the application first. Both can wait until you actually read the results before doing the work.

1. Is the data already in memory, or does a query provider supply it?
2. Should filtering and projection happen before the results are brought into the application?

IEnumerable<T> represents a sequence that can be enumerated. LINQ operations on an IEnumerable<T> normally use delegates and execute over the sequence as it is enumerated. IQueryable<T> also represents an enumerable query, but its LINQ operations can build expression trees. A query provider can inspect those expressions and translate supported operations into its own query language. Entity Framework Core is a common example when querying a database.

The practical benefit of IQueryable is that filtering, sorting, and projection can stay with the provider before materialization. This can reduce the rows, columns, data transfer, and application memory required. However, IQueryable does not automatically mean database execution or better performance. It depends on the provider and what that provider can translate.

Both forms can use deferred execution. Writing the query does not normally execute it immediately. Enumeration or a terminal operation causes execution.

A useful production pattern is to keep provider supported filtering, sorting, and projection on IQueryable until the query has the shape you need. Then materialize the results and use IEnumerable for application side processing. A common mistake is calling ToList too early, because operations after that point run against the materialized data. Another mistake is assuming every C# method can be translated by an IQueryable provider. Unsupported expressions may fail during translation or require a different query shape.

A common mistake is saying that IQueryable always means a database query. It does not. IQueryable depends on its query provider, and AsQueryable on an in memory collection still uses an in memory provider. Another mistake is assuming IQueryable is always faster. The benefit depends on provider translation, query shape, data size, indexes, and the amount of data returned. Developers can also call ToList too early, which materializes the current query and causes later LINQ operations to run in memory. Another mistake is assuming every C# method can be translated by every IQueryable provider.

Start with the practical difference: IEnumerable normally processes an existing sequence, while IQueryable builds a provider inspectable query. Then explain that both can use deferred execution. Give the Entity Framework Core example and mention that early materialization moves later work into application memory. This shows both C# understanding and production judgment.

LINQ is a set of C# and .NET query features that let me filter, transform, sort, group, and combine data in a consistent way. With LINQ over IEnumerable<T>, many operators such as Where and Select use deferred execution. They describe the work first and usually perform it when the sequence is enumerated. This matters because the source can change before enumeration, and enumerating the same query again can repeat the work.

LINQ gives a program a simple way to ask for selected information from a group of values. For example, a program can keep only active customers, choose their names, and sort those names. An important detail is that describing these steps does not always perform the work immediately. In many cases, the work happens later when the program actually reads the results. This can avoid work that is never needed, but it also means that changes made to the original values before they are read can affect the final result.

1. Should I focus on LINQ over in memory sequences using IEnumerable<T>?
2. Would you like an example of deferred execution and repeated enumeration?

LINQ means Language Integrated Query. It provides standard query operators such as Where, Select, OrderBy, GroupBy, Any, First, and ToList. These operators let C# code query sequences using a consistent style.

For IEnumerable<T>, operators such as Where and Select normally use deferred execution. Calling them creates an object that represents how values should be produced. It does not normally enumerate the source at that moment. Enumeration starts when code requests values, such as through foreach, or when an operator such as ToList consumes the sequence to produce a result immediately.

For example, suppose a List<int> contains 1, 2, and 3. I create a Where query that keeps values greater than 1. I then add 4 to the source list before enumerating the query. When the query is enumerated, it can produce 2, 3, and 4 because it reads the source at enumeration time.

If I instead call ToList before a later source change, ToList enumerates the query immediately and creates a separate List<int> containing the produced values. Later changes to the original list do not automatically change that new list.

Operator behavior also matters. Where and Select can usually process values as they are requested. An operator such as OrderBy must read and buffer its input before it can produce values in sorted order. Repeatedly enumerating a deferred query can also repeat its work.

Deferred execution is useful when results may never be needed, when early filtering can reduce later work, or when the current source state should be observed. In production code, I materialize with ToList or ToArray when I need a stable snapshot or intentionally want the query to execute once.

A common mistake is assuming every LINQ operator executes when the query expression is created. Many operators over IEnumerable<T> are deferred. Another mistake is enumerating the same deferred query several times without realizing that its work can run again each time. Developers may also assume a deferred result is a snapshot even though changes to the source before later enumeration can affect the values produced. Another mistake is assuming every operator streams values. OrderBy must consume and buffer its input before returning correctly ordered values. Calling ToList unnecessarily can also add execution and memory cost when a snapshot is not needed.

Start by defining LINQ as a consistent way to query sequences. Then explain deferred execution with one simple source change example. Make it clear that Where and Select usually describe work, enumeration performs that work, and ToList forces enumeration and stores the produced values. Mention repeated enumeration and buffering operators to show practical production understanding.

They are two ways to write the same LINQ query. Query syntax often reads more like a data request, while method syntax is the direct chain of LINQ methods and works for every operator. In C#, query syntax is usually translated by the compiler into method calls such as Where, Select, GroupBy, and Join.

This question asks about two ways to write the same LINQ query in C#. One style looks like a small request written inside the language. The other style uses normal method calls with dots. Both can get the same result from the same data. The interviewer wants to know whether I can read both forms and choose the one that is easier to understand in real code. It also checks that I know they are not two different features with different results.

1. Does your team prefer one LINQ style?
2. Are joins and groupings common in this codebase?

Query syntax is shorthand for method syntax. The C# compiler lowers query keywords into method calls when a matching translation exists. For example, from, where, select, join, group, and orderby map to methods such as Select, Where, Join, GroupBy, and OrderBy. That is why both styles usually produce the same runtime result and similar performance.

The main difference is readability. Query syntax often feels easier when the query looks like SQL, especially with joins and groupings. Method syntax often feels easier when you have a long chain of transformations, lambda expressions, or a LINQ operator that has no query keyword. Not every LINQ operator has a query form, so method syntax is the more complete and flexible style. In production, the best choice is usually the style that makes the query easiest for your team to read and maintain. The important point is that the choice is mostly about clarity, not about different results.

A common mistake is thinking query syntax is faster. It is usually just a different way to write the same query. Another mistake is assuming every LINQ operator has a query form. Some operators only exist as methods. A third mistake is forgetting that deferred execution still applies in both styles when the sequence supports it.

Say that query syntax is shorthand and method syntax is the full direct form. Then add that you choose the style that is easiest to read for that query.