How to Convert a LINQ Expression like 'obj => obj.Prop' to 'parent => parent.obj.Prop' for Nested Object Access?

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Convert LINQ expression "obj => obj.Prop" to "parent => parent.obj.Prop": step-by-step guide

LINQ expressions provide a powerful way to access the properties of an object. However, sometimes you need to traverse multiple levels of a hierarchy, such as accessing properties of nested objects.

Consider the following LINQ expression:

<code class="language-c#">cust => cust.Name</code>

This expression retrieves the Name property of the cust object. But what if you need to access the Name property of the Customer object in the CustomerModel? The original expression seems to be insufficient.

To solve this problem, a method is needed that accepts the original expression and generates a new expression, taking the parent class as an input parameter. This new expression will become the parameter of the MVC method, allowing efficient access to nested properties.

Initial release and bugs

One possible approach is:

<code class="language-c#">public Expression<Func<object>> ExpressionFromField<T, TModel>(FieldDefinition<T> field)
    where TModel : BaseModel<T>
{
    var param = Expression.Parameter(typeof(TModel), "t");
    var body = Expression.PropertyOrField(param, nameof(SelectedItem));
    var member = Expression.MakeMemberAccess(body, field.Member);
    return Expression.Lambda<Func<object>>(member, param);
}</code>

However, this version fails when accessing nested properties (e.g. cust.Address.State). The error encountered indicates that the specified member does not exist in body, and body refers to the Customer object rather than the Address object.

Solution: expression combination

The required solution lies in expression composition. Just like functions can be composed, expressions can also be composed:

<code class="language-c#">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 relies on the Replace method to replace all instances of one expression with another expression. With these methods, it is possible to construct the following expression:

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

Magic expressions can now effectively access the Name property of the Customer object in the CustomerModel. It can be used as an expression parameter of an MVC method, providing seamless access to nested properties.

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