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Avoid static members in COM visible types

TypeName

AvoidStaticMembersInComVisibleTypes

CheckId

CA1407

Category

Microsoft.Interoperability

Breaking Change

NonBreaking

A type that is specifically marked as visible to COM contains a public static method.

COM does not support static methods.

This rule ignores property and event accessors, operator overloading methods, or methods marked with either the System.Runtime.InteropServices.ComRegisterFunctionAttribute attribute or the System.Runtime.InteropServices.ComUnregisterFunctionAttribute attribute.

By default, the following are visible to COM: assemblies, public types, public instance members in public types, and all members of public value types.

For this rule to fire, an assembly-level ComRegisterFunctionAttribute must be set to false, and the class- ComRegisterFunctionAttribute set to true as the following code shows.

using System;
using System.Runtime.InteropServices; 

[assembly: ComVisible(false)] 
namespace Samples
{    
    [ComVisible(true)]
    public class MyClass
    {
        public static void DoSomething()
        {
        }
    }
}

To fix a violation of this rule, change the design to use an instance method that provides the same functionality as the static method.

It is safe to suppress a warning from this rule if a COM client does not need access to the functionality provided by the static method.

Description

The following example shows a static method that violates this rule.

Code

using System;
using System.Runtime.InteropServices;
using System.Collections.ObjectModel; 

[assembly: ComVisible(false)] 

namespace Samples
{    
    [ComVisible(true)]    
    public class Book    
    {        
        private Collection<string> _Pages = new Collection<string>();         

        public Book()        
        {        
        }         

        public Collection<string> Pages        
        {            
            get { return _Pages; }        
        }                

        // Violates this rule         
        public static Book FromPages(string[] pages)        
        {            
            if (pages == null)                
                throw new ArgumentNullException("pages");             

            Book book = new Book();             

            foreach (string page in pages)            
            {                
                book.Pages.Add(page);            
            }             return book;        
        }    
    }
}

Comments

In this example, the Book.FromPages method cannot be called from COM.

Description

To fix the violation in the previous example, you could change the method to an instance method, but that does not make sense in this instance. A better solution is to explicitly apply ComVisible(false) to the method to make it clear to other developers that the method is not visible from COM.

The following example applies ComRegisterFunctionAttribute to the method.

Code

using System;
using System.Runtime.InteropServices;
using System.Collections.ObjectModel;

[assembly: ComVisible(false)]

namespace Samples
{
    [ComVisible(true)]
    public class Book
    {
        private Collection<string> _Pages = new Collection<string>();

        public Book()
        {
        }

        public Collection<string> Pages
        {
            get { return _Pages; }
        }

        [ComVisible(false)]
        public static Book FromPages(string[] pages)
        {
            if (pages == null)
                throw new ArgumentNullException("pages");

            Book book = new Book();

            foreach (string page in pages)
            {
                book.Pages.Add(page);
            }

            return book;
        }
    }
}

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