C#GetProcAddress返回零

public class MyClass
{
    internal static class UnsafeNativeMethods
    {
        [DllImport("kernel32.dll",CharSet = CharSet.Auto,SetLastError = true)]
        internal static extern IntPtr LoadLibrary(string lpFileName);

        [DllImport("kernel32.dll",SetLastError = true)]
        internal static extern bool SetDllDirectory(string lpPathName);

        [DllImport("kernel32.dll",SetLastError = true)]
        internal static extern IntPtr GetProcAddress(IntPtr hModule,string procName);
    }

    private void MyFunc()
    {
        IntPtr _dllHandle;
        IntPtr _fptr;
        string _fullPath = ".mydll.dll";
        string _procName = "MyDllFunc";

        _dllHandle = UnsafeNativeMethods.LoadLibrary(_fullPath);
        _fptr = UnsafeNativeMethods.GetProcAddress(_dllHandle,_procName); // <-- Always returns zero.
    }
}

我确定功能名称拼写正确,_fullPath可能是正确的,因为_dllHandle总是被赋值为非零值.您可能能够提供的任何见解表示赞赏.谢谢.