I get the following error while setting an insert command onto my data adaptor (second line):

Object reference not set to an instance of an object.

adptr_DataAdapter1 = new SqlDataAdapter(str_SQL1, cnn_DataConnection1);
adptr_DataAdapter1.InsertCommand.CommandText = "INSERT INTO tbl_test (str_field1) VALUES('val1')";

Any idea why this is happening?

Create the SqlCommand object first:

SqlDataAdapter da = new SqlDataAdapter();   // Create the InsertCommand.   There are many ways to do this...   da.InsertCommand = cnn_DataConnection1.CreateCommand();   da.InsertCommand.InsertCommand = "INSERT INTO Customers (CustomerID) VALUES ('val1')";

Create the SqlCommand object first:

SqlDataAdapter da = new SqlDataAdapter();   // Create the InsertCommand.   There are many ways to do this...   da.InsertCommand = cnn_DataConnection1.CreateCommand();   da.InsertCommand.InsertCommand = "INSERT INTO Customers (CustomerID) VALUES ('val1')";