Generic constraints (where clause) in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the where clause in C# generics?

The where clause sets rules on the types that can be used as generic arguments, ensuring they meet certain requirements like implementing an interface or having a parameterless constructor.

beginner

Which of these is a valid generic constraint in C#?

Examples include where T : class (reference type), where T : struct (value type), where T : new() (has parameterless constructor), or where T : SomeInterface (implements interface).

intermediate

Explain the constraint where T : new().

It means the generic type <code>T</code> must have a public parameterless constructor, so you can create instances of <code>T</code> inside the generic class or method.

intermediate

Can you use multiple constraints on a single generic type parameter? How?

Yes. You separate constraints with commas in the where clause, for example: where T : class, ISomeInterface, new() means T must be a reference type, implement ISomeInterface, and have a parameterless constructor.

beginner

What happens if you try to use a type that does not meet the generic constraints?

The code will not compile. The compiler checks the constraints and gives an error if the type argument does not satisfy them.

What does where T : struct mean in a generic constraint?

A T must be a value type

B T must be a reference type

C T must have a parameterless constructor

D T must inherit from a class

where T : struct restricts T to value types like int, double, or structs.

Which constraint ensures the generic type has a parameterless constructor?

A where T : new()

B where T : class

C where T : struct

D where T : IDisposable

where T : new() requires a public parameterless constructor.

How do you specify that a generic type must implement an interface IMyInterface?

A where T inherits IMyInterface

B where T implements IMyInterface

C where T : IMyInterface

D where T : class

The syntax is where T : IMyInterface to require implementation.

Can you combine multiple constraints on one generic type parameter?

A No, only one constraint is allowed

B Yes, but only two constraints max

C Only if they are interfaces

D Yes, separated by commas

Multiple constraints are separated by commas in the where clause.

What will happen if you use a type that does not meet the generic constraints?

A Code compiles but throws runtime error

B Code does not compile

C Code runs but behaves unpredictably

D Code compiles with warning

The compiler enforces constraints and will not compile code that violates them.

Describe how to use the where clause to restrict a generic type to reference types that implement an interface and have a parameterless constructor.

Think about combining constraints separated by commas.

Explain why generic constraints are useful when writing generic classes or methods.

Consider what happens if you try to use a type that doesn't fit the expected pattern.