i m new in .net i hv a problem that in row updating in gridview. text box get only null value..plezzz help me. my code is

You have to use

textBox.Text

Property. I can see you only using textbox and not the text/value inside it..

So it should look like

protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
int index = GridView1.EditIndex;
        GridViewRow row = GridView1.Rows[index];
        TextBox t1 = row.FindControl("emp_name") as TextBox;
        TextBox t2 = row.FindControl("emp_add") as TextBox;
        TextBox t3 = row.FindControl("emp_mob") as TextBox;
        TextBox t4 = row.FindControl("salary") as TextBox;
        string t5= GridView1.DataKeys[e.RowIndex].Value.ToString();
       string dt = "update employee set emp_name='" + t1.Text + "',emp_add='" + t2.Text + "',emp_mob='" + t3.Text+ "',salary='" + t4.Text + "'where emp_id=" + t5.Text+ "";
        con.Open();
        cmd = new SqlCommand(dt, con);
        cmd.ExecuteNonQuery();
        GridView1.EditIndex = -1;
        con.Close();
        bind();
      
    }

after getting your comments, it looks, your textbox name is txtemp_name, but your passing emp_name in findcontrol.So its not identifying it.

Well yes, it will.

Simplfying a bit:

TextBox t1 = row.FindControl("emp_name") as TextBox;
string s = "hello: " + t1;

The second line calls an implicit ToString method on the TextBox object t1, which returns the default object ToString implementation - which is the name of the type of the object: or "System.Web.UI.WebControls.TextBox" rather than the Text content of the textbox itself.

There are three things seriously wrong with your approach:
1) If the control name is not found, FindControl will return null - which will crash your webpage. You must check for this!
2) You should use the TextBox.Text property instead of the whole control.
3) Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead, or the first person anywhere in teh world who feels like it can destroy your entire database without any difficulty.