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

Object Initializers are just syntactic sugar that requires a clever compiler, and as of the current implementation you can't call methods in the initializer.

var x = new BaseDataObject { SomeValue = 3, SomeOtherValue = 4 };

Will get compiler to something like this:

BaseDataObject tempObject = new BaseDataObject();
tempObject.SomeValue = 3;
tempObject.SomeOtherValue = 4;
BaseDataObject x = tempObject;

The difference is that there can't be any synchronization issues. The variable x get's assigned the fully assigned BaseDataObject at once, you can't mess with the object during it's initialization.

You could just call the extension method after the object creation:

var x = new BaseDataObject { SomeValue = 3, SomeOtherValue = 4 };
x.SetBarValue()

You could change SetBarValue to be a property with get/set that can be assigned during initialization:

public int BarValue
{
    set
    {
        //Value should be ignored
    }
}

Or, you could subclass / use the facade pattern to add the method onto your object:

public class DataObjectWithBarValue : BaseDataObject
{
    public void BarValue
    {
        set
        {
            SetData("bar", value);
        }
        get
        {
            (int) GetData("bar");
        }
    }
}

Answer 2

No but you could do this....:

BaseDataObject test2 = (new BaseDataObject { SomeValue = 3, SomeOtherValue = 4}).SetBarValue(5);

ANd have your extension return the object like Linq Does.

EDIT: This was a good thought untill i reread and saw that the base class was developed by a third person: aka you dont have the code. Others here have posted a correct solution.