Question

The simple demo below captures what I am trying to do. In the real program, I have to use the object initialiser block since it is reading a list in a LINQ to SQL select expression, and there is a value that that I want to read off the database and store on the object, but the object doesn’t have a simple property that I can set for that value. Instead it has an XML data store.

It looks like I can’t call an extension method in the object initialiser block, and that I can’t attach a property using extension methods.

So am I out of luck with this approach? The only alternative seems to be to persuade the owner of the base class to modify it for this scenario.

I have an existing solution where I subclass BaseDataObject, but this has problems too that don’t show up in this simple example. The objects are persisted and restored as BaseDataObject – the casts and tests would get complex.

public class BaseDataObject {      // internal data store     private Dictionary<string, object> attachedData = new Dictionary<string, object>();      public void SetData(string key, object value)     {         attachedData[key] = value;     }      public object GetData(string key)     {         return attachedData[key];     }      public int SomeValue { get; set; }     public int SomeOtherValue { get; set; }  }  public static class Extensions {     public static void SetBarValue(this BaseDataObject dataObject,                                         int            barValue)     {         /// Cannot attach a property to BaseDataObject?         dataObject.SetData('bar', barValue);     } }  public class TestDemo {      public void CreateTest()     {         // this works         BaseDataObject test1 = new BaseDataObject          { SomeValue = 3, SomeOtherValue = 4 };          // this does not work - it does not compile         // cannot use extension method in the initialiser block         // cannot make an exension property           BaseDataObject test2 = new BaseDataObject { SomeValue = 3, SomeOtherValue = 4, SetBarValue(5) };     } } 

One of the answers (from mattlant) suggests using a fluent interface style extension method. e.g.:

// fluent interface style public static BaseDataObject SetBarValueWithReturn(this BaseDataObject dataObject, int barValue) {     dataObject.SetData('bar', barValue);     return dataObject; }  // this works BaseDataObject test3 = (new BaseDataObject { SomeValue = 3, SomeOtherValue = 4 }).SetBarValueWithReturn(5); 

But will this work in a LINQ query?

Answer 1

Even better:

public static T SetBarValue<T>(this T dataObject, int barValue)         where T : BaseDataObject      {         dataObject.SetData('bar', barValue);         return dataObject;     } 

and you can use this extension method for derived types of BaseDataObject to chain methods without casts and preserve the real type when inferred into a var field or anonymous type.