try with
sqlCmd.CommandText = ("SELECT [Projektdefinition] FROM " + ((DataRowView)cbTbl.SelectedItem).Row["TABLE_NAME"].ToString());
SqlDataAdapter sqlDataAdap = new SqlDataAdapter(sqlCmd);
try
{
con.Open();
DataTable datatable= new DataTable();
sqlDataAdap.Fill(datatable);
comboBox1.DataSource = datatable;
comboBox1.DisplayMember = "Projektdefinition";
comboBox1.ValueMember = "Projektdefinition";
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}