Retrieving an Expression from a property and adding it to an expression tree

问题

I've tried to simplify this example, as the actual code I'm playing with is more complex. So while this example may seem silly, bear with me. Let's say I'm working with the AdventureWorks database and I decide I want to add a property called Blarg to the Product table that returns an expression that contains code I would like to use in several places:

public partial class Product
{
    public Expression<Func<Product, string>> Blarg
    {
        get { return product => product.ProductModelID.HasValue ? "Blarg?" : "Blarg!"; }
    }
}

What I want to do is create an expression expression tree, have it get the Expression from Product.Blarg, and group by the result. Something like this:

var productParameter = Expression.Parameter(typeof(Product), "product");

// The Problem
var groupExpression = Expression.Lambda<Func<Product, string>>(
    Expression.Invoke(
        Expression.Property(productParameter, "Blarg"), 
        productParameter), 
    productParameter);

using (AdventureWorksDataContext db = new AdventureWorksDataContext())
{
    var result = db.Products.GroupBy(groupExpression).ToList();
    // Throws ArgumentException: "The argument 'value' was the wrong type. 
    //  Expected 'System.Delegate'. 
    //  Actual 'System.Linq.Expressions.Expression`1[System.Func`2[LINQ_Test.Product,System.String]]'."
}

Obviously groupExpression is incorrect (see the code comment for the exception), but I'm not sure how I should be doing it. What I thought I was saying is "get the Expression from the product.Blarg, execute it, and return the string result." I guess that's not what I'm actually saying there, though. I'm still trying to figure out expression trees. Any idea how I could pull this off?

回答1:

When you call Expression.Invoke, the first argument must be an existing LambdaExpression - it can't be an Expression to a LambdaExpression. Or in other words: it isn't going to evaluate Product.Blarg per row and use a different sub-expression each time.

Instead, you would retrieve this lambda first, perhaps making it static and accessing it via reflection if you only know it by name:

var lambda = (LambdaExpression) typeof(Product)
          .GetProperty("Blarg").GetValue(null,null);

And pass lambda in as the argument to Expression.Invoke; here's a fully working LINQ-to-Objects example showing this (via AsQueryable()):

using System;
using System.Linq;
using System.Linq.Expressions;
public partial class Product
{
    public static Expression<Func<Product, string>> Blarg
    {
        get { return product => product.ProductModelID.HasValue ? "Blarg?" : "Blarg!"; }
    }
    public int? ProductModelID { get; set; }

    static void Main()
    {
        var lambda = (LambdaExpression)typeof(Product)
          .GetProperty("Blarg").GetValue(null, null);

        var productParameter = Expression.Parameter(typeof(Product), "product");

        // The Problem
        var groupExpression = Expression.Lambda<Func<Product, string>>(
            Expression.Invoke(
                lambda,
                productParameter),
            productParameter);

        var data = new[] {
            new Product { ProductModelID = 123},
            new Product { ProductModelID = null},
            new Product { ProductModelID = 456},
        };
        var qry = data.AsQueryable().GroupBy(groupExpression).ToList();
    }
}

回答2:

var qry = data.AsQueryable().GroupBy(Blarg).ToList();

That works, same as Marc's code.

Note: Blarg is already correct, there is no reason to 're-invoke' it.

来源：https://stackoverflow.com/questions/1599999/retrieving-an-expression-from-a-property-and-adding-it-to-an-expression-tree

标签

.net

linq-to-sql

expression-trees