How can I make an interop call using a struct as a parameter that contains a string

I am attempting to call a C++ function in a DLL from C# via interop. The DLL was written by someone else. The function signature is as follows:

AXI_DLL_EXPORT int __stdcall GetFileType(StringParam *stringParam, int *fileType)

StringParam is a struct defined as follows:

struct StringParam
{
    int size;    // 4 byte integer for the size of the string buffer
    LPWSTR buff; // Pointer to a wide char buffer
}

This struct is used for passing strings back and forth. It can be used as both in and out parameters. The purpose of the size field in the struct is primarily for returning strings to the caller and to indicate the size of the buffer required. If the size of the buffer (provided by the caller) is too small to hold the string the caller can be informed of this and can then provide a larger buffer in a successive call to the function.

In C# I created a corresponding struct as follows:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
internal struct StringParam
{
    public int Size;

    [MarshalAs(UnmanagedType.LPWStr)]
    public string Text;

    public StringParam(string text)
    {
        this.Text = text;
        this开发者_如何学JAVA.Size = text.Length;
    }
}

I declared the DllImport function call as follows:

[DllImport("Custom.dll", CharSet = CharSet.Unicode)]
public static extern int GetFileType(StringParam stringParam, out int fileType);

The call fails with the following error:

A call to PInvoke function '... GetFileType' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

How should I declare the struct and call the native function from C#?

public static extern int GetFileType(ref StringParam stringParam, out int fileType); 

Have you tried :

[DllImport("Custom.dll", CharSet = CharSet.Unicode)]
public static extern int GetFileType(StringParam stringParam, ref int fileType);

And I think you have to pin the ref int before you pInvoke.