Pass values to parameter of SP having multiple values

protected void btnOk_Click(object sender, EventArgs e)
{

string command = @"dbo.app_test";
GridView1.Visible = true;
SqlConnection con1 = connection_build();
SqlCommand cmd = new SqlCommand(command, con1);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("@appname",SqlDbType.VarChar).Value=appname_values();
cmd.Parameters.Add("@UserID", SqlDbType.VarChar).Value = userid_values();

SqlDataAdapter da = new SqlDataAdapter(cmd);
con1.Close();
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
}

protected string userid_values()
{
string userid = "";
int cnt = ddluserid.GetSelectedIndices().Count();

for (int i = 1; i <= cnt; i++)
{
if (ddluserid.Items[i].Selected)
{

if (i == cnt)
{
userid += ddluserid.Items[i].Text ;
}
else
userid += ddluserid.Items[i].Text + ",";

//if (i == cnt)
//{
// userid += "'" + ddluserid.Items[i].Text + "'";
//}
//else
// userid += "'" + ddluserid.Items[i].Text + "',";
}
}


return userid;
}

protected string appname_values()
{
string appname="";
int cnt = ddlappname.GetSelectedIndices().Count();

for (int i = 1; i <= cnt;i++ )
{
if(ddlappname.Items[i].Selected)
{

if (i == cnt)
{
appname += ddlappname.Items[i].Text ;
}
else
appname += ddlappname.Items[i].Text + ",";


//appname += "'" + ddlappname.Items[i].Text + "'";
//if (i == cnt)
//{
// appname += "'" + ddlappname.Items[i].Text + "'";
//}
//else
// appname += "'" + ddlappname.Items[i].Text + "',";
}
}


return appname;
}

Above is the code given .. Please help me out with the problem