changing default parameter of sqldatasource

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C#

ASP.NET

I have 2 ASPxComboBox controls. Based on one combo, I want to fill the other. I'm using Ajax callback concept. With that, in code behind, I'm able to get the selected value of first combo. And then, I want to know, how to bind the second combo which is connected to a SqlDataSource with select stored procedure. I want to change the default value of the SqlDataSource & then bind it. I did as follows. But unable to bind.

SqlDataSource2.SelectParameters["input4"].DefaultValue = "combo1Value";
       secondCombo.DataBind();

Thanks in advance