I believe you would need to provide full-qualified name of the assembly where the type is looked for. With the code you sent, you need provide how to locate the System.Data.dll assembly where System.Data namespace resides. StreamWriter worked for you as it is in System.dll assembly and therefore automatically checked when Type.GetType is used. I check the version of System.Data assembly using gacutil and then tried following which worked:

Type type=Type.GetType("System.Data.SqlClient.SqlCommand, System.Data, Version=1.0.3300.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, Custom=null");