Web form error has an error on Default.aspx

Question

Hi Team

 

If you have an error when creating a web form, error "Compiler Error Message: CS1061: 'default_aspx' does not contain a definition for 'btnStart_Click' and no accessible extension method 'btnStart_Click' accepting a first argument of type 'default_aspx' could be found (are you missing a using directive or an assembly reference?)

//Default.aspx
<asp:Button ID="btnStart" runat="server" OnClick="btnStart_Click" /> // Error is on this line, how to resolve this?
<asp:TextBox ID="SomeText" runat="server"></asp:TextBox>

//Default.cs
string results;
private async void btnStart_Click(object sender, EventArgs e)
{
    SaySomething();
    SomeText.Text = results;
}

async System.Threading.Tasks.Task<string> SaySomething()
{
    await System.Threading.Tasks.Task.Delay(1000);
    results = "Hello result";
    return results;
}