Is there any way to use an extension method in an object initializer block in C#

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.