第一种方法：

using System.IO;
using System.Runtime.InteropServices;

[DllImport("kernel32.dll")]
public static extern IntPtr _lopen(string lpPathName, int iReadWrite);

[DllImport("kernel32.dll")]
public static extern bool CloseHandle(IntPtr hObject);

public const int OF_READWRITE = 2;
public const int OF_SHARE_DENY_NONE = 0x40;
public readonly IntPtr HFILE_ERROR = new IntPtr(-1);
private void button1_Click(object sender, EventArgs e)
{
    string vFileName = @"c:\temp\temp.bmp";
    if (!File.Exists(vFileName))
    {
        MessageBox.Show("文件都不存在!");
        return;
    }
    IntPtr vHandle = _lopen(vFileName, OF_READWRITE | OF_SHARE_DENY_NONE);
    if (vHandle == HFILE_ERROR)
    {
        MessageBox.Show("文件被占用！");
        return;
    }
    CloseHandle(vHandle);
    MessageBox.Show("没有被占用！");
}


第二种方法：

public static bool IsFileInUse(string fileName)
 {
        bool inUse = true;

        FileStream fs = null;
        try
        {

            fs = new FileStream(fileName, FileMode.Open, FileAccess.Read,

            FileShare.None);

            inUse = false;
        }
        catch
        {

        }
        finally
        {
            if (fs != null)

                fs.Close();
        }
        return inUse;//true表示正在使用,false没有使用
}