Generics used in struct vs class

Question

Assume that we have the following struct definition that uses generics:

public struct Foo<T>
{
    public T First; 
    public T Second;

    public Foo(T first)
    {
        this.First = first;
    }

}

The compiler says

'Foo.Second' must be fully assigned before control is returned to the caller

However, if Foo is a class, then it compiles successfully.

public class Foo<T>
{
    public T First; 
    public T Second;

    public Foo(T first)
    {
        this.First = first;
    }

}

Why? Why the compiler treats them differently? Moreover if no constructor is defined in the first Foo then it compiles. Why this behaviour?

Answer 1

That's a requirement of structs in general -- it has nothing to do with generics. Your constructor must assign a value to all fields.

Note the same error happens here:

struct Foo
{
    public int A;
    public int B;

    public Foo()
    {
        A = 1;
    }
}

Answer 2

That is because a compiler rule enforces that all fields in a struct must be assigned before control leaves any constructor.

You can get your code working by doing this:

public Foo(T first)
{
    this.First = first;
    this.Second = default(T);
}

Also see Why Must I Initialize All Fields in my C# struct with a Non-Default Constructor?