How to Transform Expressions for Nested Properties in MVC Parameters?

Generate nested attribute expressions in MVC parameters

The goal is to create a method that converts an expression like "cust => cust.Name" to "parent => parent.obj.Name", where "parent" represents a child with type "T" MVC model for fields. This converted expression should be compatible as an argument to an MVC method.

Combined expression

The core of the proposed solution is to combine expressions, similar to how functions are combined. The following code demonstrates the combining mechanism:

<code class="language-csharp">public static Expression<Func<TSource, TResult>> Compose<TSource, TIntermediate, TResult>(
    this Expression<Func<TSource, TIntermediate>> first,
    Expression<Func<TIntermediate, TResult>> second)
{
    return Expression.Lambda<Func<TSource, TResult>>(
        second.Body.Replace(second.Parameters[0], first.Body),
        first.Parameters[0]);
}</code>

This method replaces the expression instance with the following code:

<code class="language-csharp">public class ReplaceVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public ReplaceVisitor(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }

    public override Expression Visit(Expression ex)
    {
        if (ex == from) return to;
        else return base.Visit(ex);
    }
}

public static Expression Replace(this Expression ex,
    Expression from,
    Expression to)
{
    return new ReplaceVisitor(from, to).Visit(ex);
}</code>

Practical example

Given an attribute selector:

<code class="language-csharp">Expression<Func<object, string>> propertySelector = cust => cust.Name;</code>

and a model selector:

<code class="language-csharp">Expression<Func<Model, Customer>> modelSelector = model => model.Customer;</code>

You can combine them as follows:

<code class="language-csharp">Expression<Func<Model, string>> magic = modelSelector.Compose(propertySelector);</code>

Using this technique, you can transform expressions to access nested properties so that they fit into MVC method parameters.

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