class CiscoSwitch
{
    Dictionary<int, CiscoVSAN> VSANList = new Dictionary<int, CiscoVSAN>();
    public void ParseFCIE(string block)
    {}
}

class CiscoVSAN
{
     Dictionary<string, CiscoSwitch> MemberSwitches = new Dictionary<string, CiscoSwitch> ();
}

ParseFCIE(string block)
{
     string DID = string.Empty;
     //partial implementation
     // 'this' is a CiscoSwitch object
     //this works
     var vsans= this.VSANList.SelectMany(v => v.Value.MemberSwitches.
                          Where(d => d.Value.switchName == this.switchName));
     // assume DID now has a value;
     // this line the compiler says the type arguments cannot be inferred from usage
     if (this.VSANList.SelectMany(v => v.Value.MemberSwitches.ContainsKey(DID)))
     {}
}

if (this.VSANList.SelectMany(v => v.Value.MemberSwitches).Any(x => x.ContainsKey(DID))) {
}