静态extern void Buffer.InternalBlockCopy是.NET框架的一部分吗?

FileStream is implemented in C# 在Read(..)方法中,我可以看到:

n = ReadCore(_buffer, 0, _bufferSize);
...
Buffer.InternalBlockCopy(_buffer, _readPos, array, offset, n);
...

buffer.cs . 方法BlockCopy被定义为static extern。这个方法是在哪里定义的?它是.NET的一部分?它是托管的还是本地的?

[System.Runtime.InteropServices.ComVisible(true)]
public static class Buffer
{
    // Copies from one primitive array to another primitive array without
    // respecting types.  This calls memmove internally.  The count and 
    // offset parameters here are in bytes.  If you want to use traditional
    // array element indices and counts, use Array.Copy.
    [System.Security.SecuritySafeCritical]  // auto-generated
    [ResourceExposure(ResourceScope.None)]
    [MethodImplAttribute(MethodImplOptions.InternalCall)]
    public static extern void BlockCopy(Array src, int srcOffset,
        Array dst, int dstOffset, int count);
..
}