Introduction

While implementing my first projects using C# I found out that there were several issues to take into account if I wanted my classes to behave correctly and make good friends with .NET. This list is more about the "hows" and the "whats" and not the "whys" and while it is in no way complete, it contains the guidelines that I currently follow. Ah, and I almost forgot... it's guaranteed to be incomplete!

The Guidelines

1. ImplementIComparable.CompareTo() if ordering of objects makes sense. Also implement operators <, <=, > and >= in terms of IComparable.CompareTo().

   int IComparable.CompareTo(object obj)
   { 
       return m_data - ((MyType)obj).m_data; 
   } 
   
   publicstaticbooloperator<(MyType lhs, MyType rhs)
   { 
       return ((IComparable)lhs).CompareTo(rhs) < 0;
   }
   
   publicstaticbooloperator<=(MyType lhs, MyType rhs)
   { 
       return ((IComparable)lhs).CompareTo(rhs) <= 0;
   }
   
   publicstaticbooloperator>(MyType lhs, MyType rhs)
   { 
       return ((IComparable)lhs).CompareTo(rhs) > 0;
   }
   
   publicstaticbooloperator>=(MyType lhs, MyType rhs)
   { 
       return ((IComparable)lhs).CompareTo(rhs) >= 0;
   }
   						

2. Implement conversion functions only if they actually make sense. Make conversions explicit if the conversion might fail (throw an exception) or if the conversion shouldn’t be abused and it’s only necessary for low level code. Only make conversions implicit if it makes the code clear and easier to use and the conversion cannot fail.

   private MyType(int data)
   { 
       m_data = data;
   }
   
   publicstaticexplicitoperatorMyType(int from)
   { 
       returnnew MyType(from);
   }
   
   publicstaticimplicitoperatorint(MyType from)
   { 
       return from.m_data;
   }
   						

3. Always implement Object.ToString() to return a significant textual representation.

   publicoverridestring ToString()
   { 
       returnstring.Format("MyType: {0}", m_data);
   }
   						

4. Implement Object.GetHashCode() and Object.Equals()if object equality makes sense. If two objects are equal (Object.Equals() returns true) they should return the same hash code and this value should be immutable during the whole lifecycle of the object. The primary key is usually a good hash code for database objects.
   For reference types implement operators == and != in terms of Object.Equals().

   publicoverrideintGetHashCode()
   { 
       return m_data.GetHashCode();
   }
   
   publicoverrideboolEquals(object obj)
   { 
       
   // Call base.Equals() only if this class derives from a 
   // class that overrides Equals()
   if(!base.Equals(obj))
          returnfalse;
   
       if(obj == null)
          returnfalse; 
   
       // Make sure the cast that follows won't failif(this.GetType() != obj.GetType())
          returnfalse;
   
       // Call this if m_data is a value type
       MyType rhs = (MyType) obj;
       return m_data.Equals(rhs.m_data);  
   
       // Call this if m_data is a reference type//return Object.Equals(m_data, rhs.m_data);
   }
   
   publicstaticbooloperator==(MyType lhs, MyType rhs)
   { 
       
   								
   if(lhs == null) 
           returnfalse; 
   
       return lhs.Equals(rhs);
   }
   
   publicstaticbooloperator!=(MyType lhs, MyType rhs)
   { 
       return !(lhs == rhs);
   }
   						

5. For value types, implement Object.Equals() in terms of a type-safe version of Equals() to avoid unnecessary boxing and unboxing.

   publicoverrideintGetHashCode()
   { 
       return m_data.GetHashCode();
   }
   
   publicoverrideboolEquals(object obj)
   { 
       
   								
   if(!(obj is MyType))
           returnfalse;
   
       returnthis.Equals((MyType) obj);
   }
   
   publicboolEquals(MyType rhs)
   {
       
   								
   // Call this if m_data is a value type  
   return m_data.Equals(rhs.m_data);          
   
       // Call this if m_data is a reference type//   
   return Object.Equals(m_data, rhs.m_data);
   }
   
   publicstaticbooloperator==(MyType lhs, MyType rhs)
   { 
       return lhs.Equals(rhs);
   }
   
   publicstaticbooloperator!=(MyType lhs, MyType rhs)
   { 
       return !lhs.Equals(rhs);
   }
   						

6. Enumerations that represent bit masks should have the [Flags] attribute.
   
   
7. All classes and public members should be documented using XML comments. Private members should be documented using normal comments. XML comments should at least include <summary>, <param> and <returns> elements.
   
   
8. If a class is just meant to be a "container" for static methods (has no state), it should declare a private parameter-less constructor so it can’t be instantiated.
   
   
9. All classes should be CLS compliant. Add an [assembly:CLSCompliant(true)] attribute in the AssemblyInfo.cs file. If it is convenient to add a non-CLS compliant public member add a [CLSCompliant(false)] attribute to it.
   
   
10. All implementation details should be declared as private members. If other classes in the same assembly need access, then declare them as internal members. Try to expose as little as possible without sacrificing usability.
    
    
11. strings are immutable objects and always create a new copy for all the mutating operations, which makes it inefficient for assembling strings. StringBuilder is a better choice for this task.
    
    
12. object.MemberWiseClone() provides shallow copying. Implement ICloneable to provide deep copy for classes. ICloneable.Clone() is usually implemented in terms of a copy constructor.

    publicMyType(MyType rhs)
    { 
        m_data = rhs.m_data;
    } 
    
    publicobjectClone()
    { 
        returnnew MyType(this); //調用拷貝構造函數
    }
    						

13. If a class represents a collection of objects implement one or more indexers. Indexers are a special kind of property so they can be read-only, write-only or read-write.

    publicobjectthis[int index]
    { 
       get { return m_data[index]; }set { m_data[index] = value; }
    }
    						

14. For a "collection" class to be used in a foreach loop it must implementIEnumerable. IEnumerable.GetEnumerator() returns a class the implements IEnumerator.

    publicclass MyCollection: IEnumerable
    { 
        public IEnumerator GetEnumerator()
        { 
            returnnewMyCollectionEnumerator(this);
        }
    }
    						

15. The IEnumerator for a "collection" class is usually implemented in a private class. An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException. If the collection is modified between MoveNext and Current, Current will return the element that it is set to, even if the enumerator is already invalidated.

    privateclass MyCollectionEnumerator: IEnumerator
    { 
        public MyCollectionEnumerator(MyCollection col)
        { 
            m_col = col;
            m_lastChanged = col.LastChanged;
        } 
    
        publicboolMoveNext()
        { 
            if(m_lastChanged != m_col.LastChanged)
                thrownew InvalidOperationException();
    
            if(++m_index >= m_col.Data.Count)
                returnfalse; 
    
            returntrue;           
        }
    
        publicvoidReset()
        { 
            if(m_lastChanged != m_col.LastChanged)
                thrownew InvalidOperationException();
    
            m_index = -1;
        }
    
        publicobjectCurrent
        { 
            get { return m_col.Data[m_index]; } 
        } 
    }
    						

16. There is no deterministic destruction in C# (gasp!), which means that the Garbage Collector will eventually destroy the unused objects. When this scenario is not ok, implement IDisposable...

    publicclass MyClass
    {
        ...
        public ~MyClass()
        {
            Dispose(false);
        }
    
        publicvoid Dispose()
        {
            
    								
    	 Dispose(true);
            GC.SuppressFinalize(this);// Finalization is now unnecessary
        }
       
        protectedvirtualvoid Dispose(bool disposing)
        {
            
    								
    	if(!m_disposed)
            {
                if(disposing)
                {
                    // Dispose managed resources
                }
             
                // Dispose unmanaged resources
            }
          
            m_disposed = true;
        }
       
        privatebool m_disposed = false;
    }
    
    						

    And use the using statement to dispose resources as soon the object goes out of scope

    using(MyClass c = new MyClass())
    {
        ...
    } // The compiler will call Dispose() on c here

17. There is no way to avoid exceptions handling in .NET. There are several strategies when dealing with exceptions:
    
    
    • Catch the exception and absorb it

      try
      {
          ...
      }
      catch(Exception ex)
      {
          Console.WriteLine("Opps! Something failed: {0}", ex.Message);
      }
      										

    • Ignore the exception and let the caller deal with it if there's no reasonable thing to do.

      publicvoid DivByZero()
      {
          int x = 1 / 0; // Our caller better be ready to deal with this!
      }
      										

    • Catch the exception, cleanup and re-throw

      try
      {
          ...
      }
      catch(Exception ex)
      {
          // do some cleanupthrow;
      }
      										

    • Catch the exception, add information and re-throw

      try
      {
          ...
      }
      catch(Exception ex)
      {
          thrownew Exception("Something really bad happened!", ex);
      }
      										

18. When catching exceptions, always try to catch the most specific type that you can handle.

    try
    {
        int i = 1 / 0;
    }
    catch(DivideByZeroException ex) // Instead of catch(Exception ex)
    {
        ...
    }
    
    						

19. If you need to define your own exceptions, derive from System.ApplicationException, not from System.Exception.
    
    
20. Microsoft's FxCop design diagnostic tool is your friend. Use it regularly to validate your assemblies.
    
    
21. The definite guide for .NET class library designers is .NET Framework Design Guidelines .