更​​新： @ taffer的回答解释了为什么你的$ C $Ç行为古怪。

Update: @taffer's answer explains why your code behaves odd.

您可以用code以下，我已经测试它;项目表现为你描述。

You can use the code below, I have tested it; Project behaves as you described.

    private void textBox1_DragDrop(object sender, DragEventArgs e)
    {
        string[] file = (string[])e.Data.GetData(DataFormats.FileDrop);
        {
            if (Path.GetExtension(file[0]) == ".url")
            {
                //Do Stuff Here
                //
            }
        }
    }

    private void textBox1_DragEnter(object sender, DragEventArgs e)
    {
        string[] file = (string[])e.Data.GetData(DataFormats.FileDrop);

        if (Path.GetExtension(file[0]) == ".url")
        {
            e.Effect = DragDropEffects.Link;
            //Do Stuff Here
        }

    }

在一个侧面说明了第二个问题：可能引发异常，使用尝试捕捉验证

On a side note for second question: Probably an exception being thrown, use try catch to verify.