Problem with struct and property in c#

in a file I defined a public struct

public struct mystruct
{
    public Double struct1;
    public Decimal struct2;
}

In another I tried to do this:

class Test
{
    mystruct my_va;

    public mystruct my_va
    {
        get { return my_va; }
        set { my_va = value; }
    }

    public Test()
    {
        my_va.struct1 = 10;
    }
}

Intellisense recognizes My_va.struct1 but compiler says

Error 1 Cannot modify the return value of 'TEST.mystruct' because it is not a variable

How to correct the syntax ?