How to convert null value into string in c# .net?

You could do this:

e.NewValues[0] == null || e.NewValues[0].ToString() == string.Empty

If e.NewValues[0] is already a string, you could just do this:

string.IsNullOrEmpty(e.NewValues[0])

Update as of C# 6, you could also use:

string.IsNullOrEmpty(e.NewValues[0]?.ToString())

Or even:

$"{e.NewValues[0]}" == string.Empty

Another (wasteful) way to do it is with a singleton with an overridden ToString and ?? (overkill but it lets me use ?? :P)

(e.NewValues[0] ?? Empty._).ToString();

The code for the singleton is here:

public sealed class Empty
{
    private static readonly Lazy<Empty> lazy =
        new Lazy<Empty>(() => new Empty());
    public override string ToString()
    {
        return "";
    }
    public static object _ { get { return lazy.Value; } }
    private Empty()
    {
    }
}

You can use this piece of code

 (e.NewValues[0] == null) ? string.Empty : e.NewValues[0].ToString()

The above code will will return the string equivalent if not null, otherwise it will return empty string.

Otherwise you can use following code. This will handle the null case.

string.IsNullOrEmpty(Convert.ToString( e.NewValues[0] )

You'll need to check that e.NewValues[0] isn't null prior to doing a .ToString() on it.

protected void grd_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
     GridViewRow row = grd.Rows[e.RowIndex];
     for (int i = 0; i <= row.Cells.Count; i++)
     {
         String str = ((TextBox)(row.Cells[i].Controls[0])).Text;
         if (!string.IsNullOrEmpty(str))
         { 
          //Your Code goes here ::
         }
     }
}