Web Forms :: Passing Through Query String Parameter?

how to pass through a query string parameter of an assignment id and then checking and validating this as a number and displaying the information returned on screen. At present I have code which displays the information on screen but this is by entering in the assignment id and by triggering the button click event, I've shown the code for this below:

.aspx
<div>
<b>Current Assignment</b>
<br />
<asp:TextBox runat="server" ID="txtAssignmentID"></asp:TextBox>
<asp:Button runat="server" Text="Get Assignment" />
<br />
<b>Date Started:</b> <asp:Label runat="server" ID="lblAssignmentStart"></asp:Label>
<br />
<b>Status:</b> <asp:Label runat="server"></asp:Label>
<br />
<b>Current Achievement Level:</b> <asp:Label runat="server" ID="lblAssignmentLevel"></asp:Label>
<br />
<b>Date Last Calculated:</b> <asp:Label runat="server" ID="lblAssignmentCalcDate"></asp:Label>
<br />
</div>
.cs
protected void cmdGetAssignment_Click(object sender, EventArgs e)
{
//Check the assignmentID is an integer
int AssignmentID = 0;
try
{
AssignmentID = TypeConv.CInt(txtAssignmentID.Text);
}
catch
{
}
if (AssignmenttID > 0)
{
//Create new TCAAssignment object by passing the ID
TCA Assignment Current Assignment = abc.TCA Assignment (AssignmentID);
lblAssignmentStart.Text = Current Assignment.dDateCreated.ToString();
lblAssignmentStatus.Text = CurrentAssignment.rTCAAssignmentStatus.dDescription;
if (!CurrentAssignment.dTCAAchievementLevel.IsNull)
{
lblAssignmentLevel.Text = CurrentAssignment.rTCAAchievementLevel.dDescription;
}
lblAssignmentCalcDate.Text = CurrentAssignment.dAchievementDate.ToString();
}
}

Code example passing through a query string parameter of an assignment id would be great.