How to edit text in textbox on click of button in datalist

protected void edit_Click(object sender, EventArgs e)
{
Button btn = (Button)sender;
string id = btn.CommandArgument;
foreach (DataListItem item in datalist.Items)
{
TextBox tb = (TextBox)item.FindControl("txt");
if (tb.Text == id)
{
tb.ReadOnly = false;
tb.Enabled = true;
}
}
}

This is my aspx.cs code
here foreach line giving error after index 1
Error is null reference
Can you help me to change this code?

Follow below steps

Add another text box in Edit template
Add a button with commandName="Edit"
in datalist add OnEditCommand="DataList_EditCommand"

in .cs page

protected void DataList_EditCommand(object source,DataListCommandEventArgs e)
{
//Do your coding
}