Refactoring Func <T> to an expression <Func <T>>

Question

I have a method that currently accepts Func<Product, string>as a parameter, but I need it to be Expression<Func<Product, string>>. Using AdventureWorks, here is an example of what I would like to do using Func.

private static void DoSomethingWithFunc(Func<Product, string> myFunc)
{
    using (AdventureWorksDataContext db = new AdventureWorksDataContext())
    {
        var result = db.Products.GroupBy(product => new
        {
            SubCategoryName = myFunc(product),
            ProductNumber = product.ProductNumber
        });
    }
}

I would like it to look something like this:

private static void DoSomethingWithExpression(Expression<Func<Product, string>> myExpression)
{
    using (AdventureWorksDataContext db = new AdventureWorksDataContext())
    {
        var result = db.Products.GroupBy(product => new
            {
                SubCategoryName = myExpression(product),
                ProductNumber = product.ProductNumber
            });
    }
}

However, the problem that I ran into is that it is myExpression(product)invalid (will not compile). After reading some other posts, I understand why. And if it weren’t for the fact that I need a variable productfor the second part of my key, I could probably say something like this:

var result = db.Products.GroupBy(myExpression);

product, (ProductNumber). , . Func, . , , , product. ?

EDIT: , :

DoSomethingWithFunc(product => product.ProductSubcategory.Name);

Answer 1

, Expression<T>, " ", . GroupBy :

// Need an explicitly named type to reference in typeof()
private class ResultType
{
     public string SubcategoryName { get; set; }
     public int ProductNumber { get; set; }|
}

private static void DoSomethingWithExpression(
    Expression<Func<Product,
    string>> myExpression)
{
    var productParam = Expression.Parameter(typeof(Product), "product");
    var groupExpr = (Expression<Func<Product, ResultType>>)Expression.Lambda(
        Expression.MemberInit(
           Expression.New(typeof(ResultType)),
           Expression.Bind(
               typeof(ResultType).GetProperty("SubcategoryName"),
               Expression.Invoke(myExpression, productParam)),
           Expression.Bind(
               typeof(ResultType).GetProperty("ProductNumber"),
               Expression.Property(productParam, "ProductNumber"))),
        productParam);
    using (AdventureWorksDataContext db = new AdventureWorksDataContext())
    {
        var result = db.Products.GroupBy(groupExpr);
    }
}

Answer 2

, .

GroupBy , , . , , . , myExpression :

private static void DoSomethingWithExpression(Expression<Func<Product, string>> myExpression)
{
    var productParam = myExpression.Parameters[0];

    ConstructorInfo constructor = ...; // Get c'tor for return type

    var keySelector = Expression.Lambda(
                          Expression.New(constructor,
                              new Expression[] {
                                  productParam.Body,
                                  ... // Expressions to init other members
                              },
                              new MethodInfo[] { ... }), // Setters for your members
                          new [] { productParam });

    using (AdventureWorksDataContext db = new AdventureWorksDataContext())
    {
        var result = db.Products.GroupBy(keySelector);

        // ...
    }
}