OptionSetValue does not contain constructor which takes one argument - silverlight

Question

I am creating a silverlight application as a web resource for CRM 2011. Now i am creating a ServiceAppointment record in DB and after creating it i want to change its Status to "reserved" instead of requested.
I googled about this and come across the examples like Close a Service Activity Through Code and Microsoft.Crm.Sdk.Messages.SetStateRequest
They all suggesting to use "SetStateRequest" and for using this i have to set the OptionSetValue like
request["State"] = new OptionSetValue(4);
But above line gives me error saying "OptionSetValue does not contain constructor which takes one argument"
BTW i am using SOAP end point of CRM 2011 service in silverlight application
Any ideas friends?
EDIT
Following is my code
var request = new OrganizationRequest { RequestName = "SetStateRequest" };
request["State"] = 3;
request["Status"] = 4;
request["EntityMoniker"] = new EntityReference() { Id = createdActivityId, LogicalName = "serviceappointment" };
crmService.BeginExecute(request,ChangeActivityStatusCallback,crmService);
And my callback function is
private void ChangeActivityStatusCallback(IAsyncResult result) {
OrganizationResponse response;
try
{
response = ((IOrganizationService)result.AsyncState).EndExecute(result);
}
catch (Exception ex)
{
_syncContext.Send(ShowError, ex);
return;
}
}

Answer 1

You must some how be referencing some other OptionSetValue class that is not the Microsoft.Xrm.Sdk one. Try appending the namespace to see if that resolves your issue:
request["State"] = new Microsoft.Xrm.Sdk.OptionSetValue(4);
Also, why are you using late bound on the SetStateRequest? Just use the SetStateRequest class:
public static Microsoft.Crm.Sdk.Messages.SetStateResponse SetState(this IOrganizationService service,
Entity entity, int state, int? status)
{
var setStateReq = new Microsoft.Crm.Sdk.Messages.SetStateRequest();
setStateReq.EntityMoniker = entity.ToEntityReference();
setStateReq.State = new OptionSetValue(state);
setStateReq.Status = new OptionSetValue(status ?? -1);
return (Microsoft.Crm.Sdk.Messages.SetStateResponse)service.Execute(setStateReq);
}

Answer 2

Thanks Daryl for you time and effort. I have solved my problem with the way u have suggested.
I am posting my code that worked for me.
var request = new OrganizationRequest { RequestName = "SetState" };
request["State"] = new OptionSetValue { Value = 3 };
request["Status"] = new OptionSetValue { Value = 4 };
request["EntityMoniker"] = new EntityReference() { Id = createdActivityId, LogicalName = "serviceappointment" };
crmService.BeginExecute(request,ChangeActivityStatusCallback,crmService);
private void ChangeActivityStatusCallback(IAsyncResult result) {
OrganizationResponse response;
try
{
response = ((IOrganizationService)result.AsyncState).EndExecute(result);
}
catch (Exception ex)
{
_syncContext.Send(ShowError, ex);
return;
}
}