SoapHttpClientProtocol.Invoke method problem

[Cheung Wai Lun]

I have to call a web services and i only know the method name at
runtime. I create a proxy class and then modify the method:

public System.Data.DataSet GetStaffDataX() {
object[] results = this.Invoke(sMethodName, new object[] {
.....}

sMethodName is the variable to store the method name.

however when the sMethodName is not the same as the function name (i.e.
sMethodName <> GetStaffDataX), exception is throw:
[MethodName] Web Service method name is not valid

Can anyone help me?