我将重新排列代码，以便您可以确定发生了什么。您的代码有两个openFileDialog.ShowDialog()语句。很可能错过了返回ShowDialog()。
尝试类似于：

private void locbtn_Click(object sender, EventArgs e)
{
    string fileTextDefault = "Please Select Another Folder!";
    TestBox1.Text = fileTextDefault;

    OpenFileDialog openFileDialog1 = new OpenFileDialog();
    openFileDialog1.Title = "Select Folder";
    openFileDialog1.InitialDirectory = @"C:\";//--"C:\\";
    openFileDialog1.ValidateNames = false;
    openFileDialog1.CheckFileExists = false;
    openFileDialog1.CheckPathExists = true;
    openFileDialog.Filter = "All Files (*.*)|*.*";
    
    if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
    {
        var filePath = openFileDialog1.FileName;
        if (filePath != "")
        {
            textBox1.Text = filePath;
        }
    }
    else
    {
      //Do something here to indicate failure
    }
}