Assign new object to class property C# Expression Tree

Question

How one can assign the following statement using expression Tree

myFoo.myBar = new Bar();

My Code is as follows -

    public static Action<TObject, TProperty>CreateNewObjectAndSet<TObject,TProperty>(string propertyName)
    {

        ParameterExpression paramExpression = Expression.Parameter(typeof(TObject));
        MemberExpression propertyGetterExpression = Expression.Property(paramExpression, propertyName);

        var newObject = Expression.New(typeof(TProperty));

        var x = Expression.Assign(propertyGetterExpression, newObject);

        var paramExpressions = new ParameterExpression[2];
        paramExpressions[0] = paramExpression;
        paramExpressions[1] = newObject;

        Action<TObject, TProperty> result = Expression.Lambda<Action<TObject, TProperty>>(x, paramExpressions).Compile();

        return result;
    }

Compile error occurs at the statement

paramExpression[1] = newObject;

Evk · Accepted Answer · 2018-03-05 05:37:21Z

Since target expression is:

myFoo.myBar = new Bar();

you don't need 2 parameters, you only need 1 - instance of myFoo to set property on. So change your code like this:

public static Action<TObject> CreateNewObjectAndSet<TObject, TProperty>(string propertyName) where TProperty: new() {

    ParameterExpression paramExpression = Expression.Parameter(typeof(TObject));
    MemberExpression propertyGetterExpression = Expression.Property(paramExpression, propertyName);

    var newObject = Expression.New(typeof(TProperty));

    var x = Expression.Assign(propertyGetterExpression, newObject);

    var paramExpressions = new ParameterExpression[1];
    paramExpressions[0] = paramExpression;

    Action<TObject> result = Expression.Lambda<Action<TObject>>(x, paramExpressions).Compile();

    return result;
}

Then call like this:

var setter = CreateNewObjectAndSet<Foo, Bar>("myBar");
setter(myFoo);

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