# Generic type parameters (C# Programming Guide)

In a generic type or method definition, a type parameter is a placeholder for a specific type that a client specifies when they create an instance of the generic type. A generic class, such as GenericList<T> listed in Introduction to Generics, cannot be used as-is because it is not really a type; it is more like a blueprint for a type. To use GenericList<T>, client code must declare and instantiate a constructed type by specifying a type argument inside the angle brackets. The type argument for this particular class can be any type recognized by the compiler. Any number of constructed type instances can be created, each one using a different type argument, as follows:

GenericList<float> list1 = new GenericList<float>();
GenericList<ExampleClass> list2 = new GenericList<ExampleClass>();
GenericList<ExampleStruct> list3 = new GenericList<ExampleStruct>();


In each of these instances of GenericList<T>, every occurrence of T in the class is substituted at run time with the type argument. By means of this substitution, we have created three separate type-safe and efficient objects using a single class definition. For more information on how this substitution is performed by the CLR, see Generics in the Run Time.

## Type parameter naming guidelines

• Do name generic type parameters with descriptive names, unless a single letter name is completely self explanatory and a descriptive name would not add value.

public interface ISessionChannel<TSession> { /*...*/ }
public delegate TOutput Converter<TInput, TOutput>(TInput from);
public class List<T> { /*...*/ }

• Consider using T as the type parameter name for types with one single letter type parameter.

public int IComparer<T>() { return 0; }
public delegate bool Predicate<T>(T item);
public struct Nullable<T> where T : struct { /*...*/ }

• Do prefix descriptive type parameter names with "T".

public interface ISessionChannel<TSession>
{
TSession Session { get; }
}

• Consider indicating constraints placed on a type parameter in the name of parameter. For example, a parameter constrained to ISession may be called TSession.

The code analysis rule CA1715 can be used to ensure that type parameters are named appropriately.