There are certain scenarios where Textbox webcontrols will not post the Text value enter by the user back to the server. To understand these scenarios I created the following form consisting of 4 textboxes. On the first textbox (ReadOnlyTextBox) I set the ReadOnly property to true using the properties window. On the second textbox (DisabledTextBox) I set the enabled property to false using the properties window. On the next two textboxes I did not set the ReadOnly or Eanbled properties, but I did add attributes via C# code in the Form_Load event as will be highlighted later. Here is the complete form.

 
<p>
    <asp:TextBox ID="ReadOnlyTextBox" ClientIDMode="Static" runat="server" ReadOnly="True"></asp:TextBox>
</p>
<p>
    <asp:TextBox ID="DisabledTextBox" ClientIDMode="Static" runat="server" Enabled="False"></asp:TextBox>
</p>
<p>
    <asp:TextBox ID="ReadOnlyByAttributeTextBox" ClientIDMode="Static" runat="server"></asp:TextBox>
</p>
<p>
    <asp:TextBox ID="DisabledByAttributeTextBox" ClientIDMode="Static" runat="server"></asp:TextBox>
</p>
<p>
    <asp:Button ID="SubmitButton" runat="server" OnClick="SubmitButton_Click" Text="Submit" /> 
    <asp:Button ID="ClearButton" runat="server" Text="Clear Fields" OnClick="ClearButton_Click" />
</p>
<p>
    <asp:Button ID="PopulateFieldsButton" runat="server" Text="Populate Fields (ServerSide)" OnClick="PopulateFieldsButton_Click" />
    <input type="button" value="Populate Fields (ClientSide)" onclick="Populate();" />
</p>
<asp:Literal ID="OutputDisplay" runat="server"></asp:Literal>

I also added a basic javascript function to the page to set the Text value of the 4 text boxes for comparing the difference in postback results when the textboxes are populated via server side code or client side code.

<script type="text/javascript">
    function Populate() {
        var readOnly = document.getElementById("ReadOnlyTextBox");
        var disabled = document.getElementById("DisabledTextBox");
        var readOnly2 = document.getElementById("ReadOnlyByAttributeTextBox");
        var disabled2 = document.getElementById("DisabledByAttributeTextBox");

        readOnly.value = "ReadOnly By Property";
        disabled.value = "Disabled By Property";
        readOnly2.value = "ReadOnly By Atribute";
        disabled2.value = "Disabled By Attribute";
    }
</script>

Here is the C# code for the ASP.NET page. In the Page_Load event the readonly and disabled attributes are set for the relevant two textboxes. When the form is submitted the values of the tesxtboxes are read and displayed in a Literal webcontrol. There is also a PopulateFieldsButton_Click event to populate the tesxtboxes from server side.

protected void Page_Load(object sender, EventArgs e)
{
    ReadOnlyByAttributeTextBox.Attributes.Add("readonly", "readonly");
    DisabledByAttributeTextBox.Attributes.Add("disabled", "disabled");

	// uncomment below to compare effect of populating TextBoxes in form load at server side
    //SetTextBoxes();
}
      
protected void SubmitButton_Click(object sender, EventArgs e)
{
    var output = new StringBuilder();
    output.Append("Posted Values<br/>");
    output.AppendFormat("ReadOnly By Property TextBox: {0}<br/>", ReadOnlyTextBox.Text);
    output.AppendFormat("Disabled By Property TextBox: {0}<br/>", DisabledTextBox.Text);
    output.AppendFormat("ReadOnly By Attribute TextBox: {0}<br/>", ReadOnlyByAttributeTextBox.Text);
    output.AppendFormat("Disabled By Attribute TextBox: {0}<br/>", DisabledByAttributeTextBox.Text);
    OutputDisplay.Text = output.ToString();
}

protected void PopulateFieldsButton_Click(object sender, EventArgs e)
{
    SetTextBoxes();
}

protected void ClearButton_Click(object sender, EventArgs e)
{
    DisabledTextBox.Text = String.Empty;
    ReadOnlyTextBox.Text = String.Empty;
    ReadOnlyByAttributeTextBox.Text = String.Empty;
    DisabledByAttributeTextBox.Text = String.Empty;
}

private void SetTextBoxes()
{
    ReadOnlyTextBox.Text = "ReadOnly By Property";
    DisabledTextBox.Text = "Disabled By Property";
    ReadOnlyByAttributeTextBox.Text = "ReadOnly By Attribute";
    DisabledByAttributeTextBox.Text = "Disabled By Attribute";
}

On to the tests.
Test 1 – Populate server side.

• Ensure the fields are empty by clicking the Clear Fields button
• Click the Populate Fields (ServerSide)
• Click the Submit Button

The results show the Text value was posted back to all except the textbox with the disabled attribute set by C# code.

ReadOnly By Property TextBox: ReadOnly By Property
Disabled By Property TextBox: Disabled By Property
ReadOnly By Attribute TextBox: ReadOnly By Attribute
Disabled By Attribute TextBox: 

Test 2 – Populate client side

• Ensure the fields are empty by clicking the Clear Fields button
• Click the Populate Fields (ClientSide)
• Click the Submit Button

The results show the Text value was posted back to the server only on the textbox with the readoly attribute set by C# code.

ReadOnly By Property TextBox: 
Disabled By Property TextBox: 
ReadOnly By Attribute TextBox: ReadOnly By Atribute
Disabled By Attribute TextBox: 

So, if you want the Text value of the textbox to be posted to the server and you do not want the user to modify the contents of the textbox use C# code to set the ReadOnly attribute.