Unable to cast object of type system.DBNull to type system.string

Last week, I got this error when I was working on my web apps.

The code which has error:

protected string returnURL(object paramID)

{

string m_paramID;

m_paramID = (string)paramID;

string retURL = "";

if (m_paramID.Contains("IWA"))

retURL = "IWTitleDetail.aspx?Identifier=" + paramID;

else

retURL = "MsTitleDetail.aspx?Identifier=" + paramID;

return retURL;

}

Then, I changed to this and it solved the problem:

protected string returnURL(object paramID)

{

string retURL = "";

string m_paramID;

if (paramID == DBNull.Value)

{ }

else

{

m_paramID = (string)paramID;

if (m_paramID.Contains("IWA"))

retURL = "IWTitleDetail.aspx?Identifier=" + paramID;

else

retURL = "MsTitleDetail.aspx?Identifier=" + paramID;

}

return retURL;

}

Happy Coding!!