How to Convert a LINQ Expression from 'obj => obj.Prop' to 'parent => parent.obj.Prop'?

Convert LINQ expression "obj => obj.Prop" to "parent => parent.obj.Prop"

In some cases, it is necessary to convert a LINQ expression that selects a property directly from an object to an expression that selects a property through a parent object. For example, an expression like "cust => cust.Name" needs to be converted to "parent => parent.Customer.Name".

For this, you can use the concept of expression composition. This involves creating a function that accepts two expressions as arguments: one that selects a property, and another that selects the object that contains that property. The following code example provides a solution using this method:

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

Additionally, a helper method called "Replace" is required to replace all occurrences of an expression:

<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>

Using these helper methods, it is now possible to combine expressions like this:

<code class="language-csharp">// 从对象中选择一个属性
Expression<Func<Customer, string>> propertySelector = cust => cust.Name;

// 从模型中选择对象
Expression<Func<Model, Customer>> modelSelector = model => model.Customer;

// 组合两个表达式
Expression<Func<Model, string>> magic = modelSelector.Compose(propertySelector);</code>

This approach provides a flexible solution for combining expressions to access nested properties, making it suitable for a variety of scenarios, including MVC method parameters.

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