String marshalling

C# course · Expert · Lesson 16/47

Learn to pass managed strings to native APIs.

String marshalling

To pass a string to a native API you specify the marshal type: [DllImport("x.dll", CharSet = CharSet.Ansi)] for ANSI, CharSet.Unicode for UTF-16, CharSet.Auto for platform default.

You can also convert a single parameter with [MarshalAs(UnmanagedType.LPStr)]. The right type must match the encoding the function expects; otherwise the data is corrupted and the output becomes unreadable.

StringBuilder for out parameters

When a native function writes a string back (e.g. GetWindowText), you use StringBuilder. [DllImport("user32.dll")] static extern int GetWindowText(IntPtr h, StringBuilder s, int max); pre-allocates the buffer and marshals it on return.

Setting StringBuilder capacity is mandatory; otherwise the result is truncated. This is the standard way to talk to fixed-length C buffers.

Exercise Call puts with a CharSet

Declare [DllImport("libc.so.6", CharSet = CharSet.Ansi)] static extern int puts(string s);. In Main call puts("Hello"); and then write Console.WriteLine("Done");.

Show example solution

puts("Hello"); Console.WriteLine("Done");
Example solution:
using System;
using System.Runtime.InteropServices;

class Program
{
    [DllImport("libc.so.6", CharSet = CharSet.Ansi)]
    static extern int puts(string s);

    static void Main()
    {
        puts("Hello");
        Console.WriteLine("Done");
    }
}

Test yourself

Match the CharSet types

CharSet.Ansi

ANSI (1 byte)
CharSet.Unicode

UTF-16
CharSet.Auto

Platform default
UnmanagedType.LPStr

ANSI string pointer
Which CharSet uses UTF-16 strings?
- CharSet.Unicode
- CharSet.None
- CharSet.Auto
- CharSet.Ansi
Show answer

CharSet.Unicode — Correct. CharSet.Unicode uses UTF-16.

Specify the CharSet

[DllImport("x.dll", ____ = CharSet.Unicode)] static extern void Greet(string s);

Show answer

[DllImport("x.dll", CharSet = CharSet.Unicode)] static extern void Greet(string s);

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String marshalling