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Hydrating Objects With Expression Trees - Part I

LINQ With C# (Portuguese)

After my post about dumping objects using expression trees, I’ve been asked if the same could be done for hydrating objects.

Sure it can, but it might not be that easy.

What we are looking for is a way to set properties on objects of an unknown type. For that, we need to generate methods to set each property of the objects.

Such methods would look like this expression:

Expression<Action<object, object>> expression = (o, v) => ((SomeType)o).Property1 = (PropertyType)v;

Unfortunately, we cannot use the .NET Reflector trick because, if you try to compile this, you’ll get this error:

error CS0832: An expression tree may not contain an assignment operator

Fortunately, that corresponds to a valid .NET expression tree. We just have to build it by hand.

So, for a given type, the set of property setters would be built this way:

var compiledExpressions = (from property in objectType.GetProperties()
                           let objectParameterExpression = Expression.Parameter(typeof(object), "o")
                           let convertedObjectParameteExpressionr = Expression.ConvertChecked(objectParameter, objectType)
                           let valueParameter = Expression.Parameter(propertyType, "v")
                           let convertedValueParameter = Expression.ConvertChecked(valueParameter, property.PropertyType)
                           let propertyExpression = Expression.Property(convertedObjectParameter, property)
                                Expression.Lambda<Action<object, object>>(

And hydrating objects would be like this:

for (int o = 0; o < objects.Length; o++)
    var objectProperties = objects[o];

    var newObject = newObjects[o] = Activator.CreateInstance(objectType);

    for (int p = 0; p < compiledExpressions.Length; p++)
        compiledExpressions[p](newObject, objectProperties[p]);


  • Hydrating Objects With Expression Trees - Part II &nbsp;
    In my previous post I showed how to hydrate objects by creating instances and setting properties in those instances.
    But, if the intent is to hydrate the objects from data, why not having an expression that does just that? That’s what the member initialization expression is for.

  • it was came up with rss aggregation , it is removed now

  • @sandy,

    I don't mind my blogs being aggregated in other sites, as long as its origin is mentioned.

  • Hydrating Objects With Expression Trees - Part III
    To finalize this series on object hydration, I’ll show some performance comparisons between the different methods of hydrating objects.
    Code samples for this series of posts (and the one about object dumping with expression trees) can be found on my MSDN Code Gallery: Dump And Hydrate Objects With Expression Trees

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