How to Compose Expressions to Transform 'obj => obj.Prop' into 'parent => parent.obj.Prop'?

Combine expressions to convert "obj => obj.Prop" to "parent => parent.obj.Prop"

The existing expression "obj => obj.Prop" can be converted into a new expression "parent => by combining the original expression with an expression that extracts "obj" from "parent" parent.obj.Prop".

Custom expression combination function

To create combinations, functions can be combined using custom expressions:

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

This function combines two expressions by replacing references to the parameters of the second expression with the body of the first expression.

Expression replacement

To replace an expression, you can use the custom expression accessor:

<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>

This accessor replaces all occurrences of one expression with another expression.

Combined expression

Using the functions above, it is possible to combine the original expression and the "obj" extracted expression:

<code class="language-csharp">Expression<Func<Customer, object>> propertySelector = cust => cust.Name;
Expression<Func<CustomerModel, Customer>> modelSelector = model => model.Customer;
Expression<Func<CustomerModel, object>> magic = modelSelector.Compose(propertySelector);</code>

The generated expression "magic" will now select the "Name" property from the "Customer" in the "CustomerModel".

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