Need help with GPI command & function

Hi，everyone.I am learning about how to build a GPI dll. And I am Not familiar with how to create commands.

I have finished some work on commands/step_1 of demos , which has shown as below

// register the name of the command
strcpy_s(commandInfo->szName, "IF2");

// register the name of the function to call
strcpy_s(commandProperties->szProcFctName, "TC2");
commandProperties->cAutoseq = eGPIREGISTER;
commandProperties->nParam = 1;
commandProperties->aParam[0].cParamType = eGPIVARTYPEDOUBLE;
strcpy_s(commandProperties->aParam[0].szParamName, "myResult");

And

GPIEXTERNC void DLLEXPORT  TC2(TGPIDouble out1)
{

out1=10.0;
}

After registration of the dll,When I am trying to call TC2 in Scripts like below ,I get nothing on X.

DIM X
IF2(X)

Many thx for any help or hints!!
Best regards ,Q