I have a method that currently takes a Func<Product, string>
as a parameter, but I need it to be an Expression<Func<Product, string>>
. Using AdventureWorks, here's an example of what I'd like to do using the 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 I'm running into is that myExpression(product)
is invalid (won't compile). After reading some other posts I understand why. And if it wasn't for the fact that I need the product
variable for the second part of my key I could probably say something like this:
var result = db.Products.GroupBy(myExpression);
But I do need the product
variable because I do need the second part of the key (ProductNumber). So I'm not really sure what to do now. I can't leave it as a Func because that causes problems. I can't figure out how to use an Expression because I don't see how I could pass it the product
variable. Any ideas?
EDIT: Here's an example of how I would call the method:
DoSomethingWithFunc(product => product.ProductSubcategory.Name);
There's no way to splice an expression tree that is represented as an Expression<T>
object into a middle of a "tree literal" represented by lambda expression. You'll have to construct an expression tree to pass to GroupBy
manually:
// 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);
}
}