C#

[DllImport("Kernel32", CharSet = CharSet.Auto)]
static extern Int32 GetShortPathName(
String path,                // input string
StringBuilder shortPath,    // output string
Int32 shortPathLength);     // StringBuilder.Capacity
public static string ConvertLongPathToShort(string longPathName)
{
    StringBuilder shortNameBuffer;
    int size;

    shortNameBuffer = new StringBuilder();
    size = GetShortPathName(longPathName, shortNameBuffer, shortNameBuffer.Capacity);

    if(size >= shortNameBuffer.Capacity)
    {
        shortNameBuffer.Capacity = size + 1;
        GetShortPathName(longPathName, shortNameBuffer, shortNameBuffer.Capacity);
    }

    return shortNameBuffer.ToString();
}