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

obj.Prop" to "parent => parent.obj.Prop"? " /> Refactor "obj => obj.Prop" to "parent => parent.obj.Prop"?

Refactor the LINQ expression "obj => obj.Prop" into "parent => parent.obj.Prop"

When modifying an existing LINQ expression of type Expression<Func<object>> to accept a parent class, challenges arise when the expression involves nested properties. The existing ExpressionFromField<TModel> method has problems handling expressions like cust => cust.Address.State because it cannot explicitly access the intermediate object (cust.Address in this case).

The solution lies in expression composition, which combines multiple expressions into one. Here’s the step-by-step method:

1. Expression combination method

Define a static method named Compose<T, TIntermediate, TResult> that accepts three expression delegates:

<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)
{
    // 用第二个表达式的参数替换第一个表达式的参数出现
    return Expression.Lambda<Func<T, TResult>>(
        second.Body.Replace(second.Parameters[0], first.Body),
        first.Parameters[0]);
}</code>

2. Expression replacement

Creates a class named ReplaceVisitor that is derived from ExpressionVisitor:

<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);
    }
}</code>

Add an extension method Expression to the Replace class:

<code class="language-csharp">public static Expression Replace(this Expression ex,
    Expression from,
    Expression to)
{
    return new ReplaceVisitor(from, to).Visit(ex);
}</code>

3. Application combination

With the combination and substitution methods in place, you can now construct new expressions:

<code class="language-csharp">// 选择属性的表达式：
Expression<Func<Customer, object>> propertySelector =
    cust => cust.Name;

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

// 组合表达式
Expression<Func<Model, object>> magic = modelSelector.Compose(propertySelector);</code>

By combination...

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