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Generic methods (C# programming guide)

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A generic method is a method that is declared with type parameters, as follows:

static void Swap<T>(ref T lhs, ref T rhs)
{
    T temp;
    temp = lhs;
    lhs = rhs;
    rhs = temp;
}

The following code example shows one way to call the method by using int for the type argument:

public static void TestSwap()
{
    int a = 1;
    int b = 2;

    Swap<int>(ref a, ref b);
    System.Console.WriteLine(a + " " + b);
}

You can also omit the type argument and the compiler will infer it. The following call to Swap is equivalent to the previous call:

Swap(ref a, ref b);

The same rules for type inference apply to static methods and instance methods. The compiler can infer the type parameters based on the method arguments you pass in; it cannot infer the type parameters only from a constraint or return value. Therefore type inference does not work with methods that have no parameters. Type inference occurs at compile time before the compiler tries to resolve overloaded method signatures. The compiler applies type inference logic to all generic methods that share the same name. In the overload resolution step, the compiler includes only those generic methods on which type inference succeeded.

Within a generic class, non-generic methods can access the class-level type parameters, as follows:

class SampleClass<T>
{
    void Swap(ref T lhs, ref T rhs) { }
}

If you define a generic method that takes the same type parameters as the containing class, the compiler generates warning CS0693 because within the method scope, the argument supplied for the inner T hides the argument supplied for the outer T. If you require the flexibility of calling a generic class method with type arguments other than the ones provided when the class was instantiated, consider providing another identifier for the type parameter of the method, as shown in GenericList2<T> in the following example.

class GenericList<T>
{
    // CS0693.
    void SampleMethod<T>() { }
}

class GenericList2<T>
{
    // No warning.
    void SampleMethod<U>() { }
}

Use constraints to enable more specialized operations on type parameters in methods. This version of Swap<T>, now named SwapIfGreater<T>, can only be used with type arguments that implement IComparable<T>.

void SwapIfGreater<T>(ref T lhs, ref T rhs) where T : System.IComparable<T>
{
    T temp;
    if (lhs.CompareTo(rhs) > 0)
    {
        temp = lhs;
        lhs = rhs;
        rhs = temp;
    }
}

Generic methods can be overloaded on several type parameters. For example, the following methods can all be located in the same class:

void DoWork() { }
void DoWork<T>() { }
void DoWork<T, U>() { }

You can also use the type parameter as the return type of a method. The following code example shows a method that returns an array of type T:

T[] Swap<T>(T a, T b)
{
    return [b, a];
}

C# Language Specification

For more information, see the C# Language Specification.

See also