Passing `null` reference for a `ref struct` parameter in interop method

Question

I am using C# to call a DLL function.

[DllImport("MyDLL.dll", SetLastError = true)]
public static extern uint GetValue(
        pHandle handle,
        ref somestruct a,
        ref somestruct b);

How can I pass a null reference for argument 3?

When I try, I am getting a compile-time error:

Cannot convert from <null> to ref somestruct.

I also tried IntPtr.Zero.

Answer 1

You have two options:

1. Make somestruct a class, and change the function signature to:

   [DllImport("MyDLL.dll", SetLastError = true)]
   public static extern uint GetValue(
       pHandle handle, somestruct a, somestruct b);
   

   Usually this must not change anything else, except that you can pass a null as the value of a and b.

2. Add another overload for the function, like this:

   [DllImport("MyDLL.dll", SetLastError = true)]
   public static extern uint GetValue(
       pHandle handle, IntPtr a, IntPtr b);
   

   Now you can call the function with IntPtr.Zero, in addition to a ref to an object of type somestruct:

   GetValue(myHandle, ref myStruct1, ref myStruct2);
   GetValue(myHandle, IntPtr.Zero, IntPtr.Zero);