如何确定字符串变量的值是否在C＃中更改？

// holds a copy of the previous value for comparison purposes
private string oldString = string.Empty; 

private void button6_Click(object sender, EventArgs e)
{
    // Get the new string value
    string newString = //some varying value I get from other parts of my program

    // Compare the old string to the new one
    if (oldString != newString)
    {
        // The string values are different, so update the ListBox
        listBox1.Items.Clear();
        listBox1.Items.Add(x + /*other things*/);   
    }

    // Save the new value back into the temporary variable
    oldString = newString;
}

string last = string.Empty;
private void button6_Click(object sender, EventArgs e)
    {
        string x = //some varying value I get from other parts of my program
        if(x!=last)
        {
            listBox1.Items.Clear();
            listBox1.Items.Add(x + /*other things*/);
            last = x;
        }
    }

public class StringValue
{
   private bool _changed;
   public string StrValue{get; set{ _changed = true;}
   public bool Changed{get;set;}
}

string _x = string.Empty;
public string X
{
   set
   {
      if(value != this._x)
      {
         DoFancyListBoxWork();
         this._x = value;
      }
   }

   get
   {
      return this._x;
   }
}

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
    ViewState["CurrentValue"] = Your Value;
}
}
protected void btnSubmit_click(object sender, EventArgs e)
{
if (NewValue==  ViewState["CurrentValue"].ToString())
{
    lblmsg.Text = "value is not changed..";
return;
}
else
    lblmsg.Text = "value is changed..";
}