How Does Generic Constraint Nullability Affect C# Method Overloading?

Constraints and Overloading

In C#, generic constraints can be applied to type parameters to restrict the types that can be used. However, when a constraint is placed on a parameter, it affects the overload resolution process.

Consider the following code:

static void Foo<T>(T a) where T : struct { } // 1

static void Foo<T>(T? a) where T : struct { } // 2

These two functions overload Foo and are differentiated based on the nullability of T. However, attempting to add a third overload with a constraint on class types fails:

static void Foo<T>(T a) where T : class { } // 3

This is because the parameters of this function match the parameters of Foo(T a) where T : struct. To work around this, we must place the constraint on a different parameter:

class RequireStruct<T> where T : struct { }
class RequireClass<T> where T : class { }

static void Foo<T>(T a, RequireStruct<T> ignore = null) where T : struct { } // 1
static void Foo<T>(T? a) where T : struct { } // 2
static void Foo<T>(T a, RequireClass<T> ignore = null) where T : class { } // 3

Now, Foo can be overloaded to handle all three cases:

• Plain value types (e.g. int) are mapped to Foo(T a, RequireStruct ignore = null) where T : struct.
• Nullable value types (e.g. int?) are mapped to Foo(T? a) where T : struct.
• Reference types (e.g. string) are mapped to Foo(T a, RequireClass ignore = null) where T : class.

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