Question

I’ve seen this quite a few times while using Office Interop classes

this.CustomXMLParts.Add(MyResources.Data, new Office.CustomXMLSchemaCollection()); 

If I hover over the CustomXMLSchemaCollection class, it shows up as an interface. Then how come I can do a new on it ? What gives? BTW this code compiles and works.

Answer 1

You are not creating an instance of the CustomXMLSchemaCollection interface but an instance of the CustomXMLSchemaCollectionClass coclass.

The definition for CustomXMLSchemaCollection interface is:

[Guid('000CDB02-0000-0000-C000-000000000046')] [CoClass(typeof(CustomXMLSchemaCollectionClass))] public interface CustomXMLSchemaCollection : _CustomXMLSchemaCollection { } 

This means that the designated coclass that implements the interface is CustomXMLSchemaCollectionClass. My guess is that when the C# compiler sees the new for CustomXMLSchemaCollection interface it translates it to create a COM instance of the CustomXMLSchemaCollectionClass based on the attributes provided with the interface.

After writing this simple example:

namespace ConsoleApplication2 {     using System;     using Office = Microsoft.Office.Core;      class Program     {         static void Main(string[] args)         {             Office.CustomXMLSchemaCollection test = new Office.CustomXMLSchemaCollection();         }     } } 

I just ran ildasm and get the following MSIL:

.method private hidebysig static void  Main(string[] args) cil managed {   .entrypoint   // Code size       8 (0x8)   .maxstack  1   .locals init ([0] class [Interop.Microsoft.Office.Core]Microsoft.Office.Core.CustomXMLSchemaCollection test)   IL_0000:  nop   IL_0001:  newobj     instance void [Interop.Microsoft.Office.Core]Microsoft.Office.Core.CustomXMLSchemaCollectionClass::.ctor()   IL_0006:  stloc.0   IL_0007:  ret } // end of method Program::Main 

As you can see the class that is constructed is CustomXMLSchemaCollectionClass to prove my initial assumption.