How do I create an dynamic expression like ( x, y ) => DateTime.Compare( y.EFFECTIVE_DATE, x.EFFECTIVE_DATE )

Question

I am a newer coder and am having a lot of issues creating the following lamdba expression dynamically using expression trees.

( x, y ) => DateTime.Compare( y.EFFECTIVE_DATE, x.EFFECTIVE_DATE )

I am just starting to understand the basics of the Expression Trees but don't seem to know enough to create this. I don't have any good code to really put here because honestly I am really fumbling around with this. Any help would be great.

Thank you in advance.

Sorry, first time posting and having some issues with formatting items.

So, In my main method I have the following:

var sortExpression = testing.GetExpression( "EFFECTIVE_DATE", "EFFECTIVE_DATE" );

        if (e.SortDirection == SortDirection.Ascending)
        {
            paymentAuthorizationList.Sort( sortExpression );  
        }

And, in my Get Expression Method:

    public Expression<Func<AuthorizePayments, AuthorizePayments, int >> GetExpression<t>( string propertyName, string propertyValue )
    {

        var paramX = Expression.Parameter( typeof( AuthorizePayments ), "x" ); // x
        var paramY = Expression.Parameter( typeof( AuthorizePayments ), "y" ); // y

        var xDate = Expression.Property( paramX, "EFFECTIVE_DATE" ); // x.EFFECTIVE_DATE
        var yDate = Expression.Property( paramY, "EFFECTIVE_DATE" ); // y.EFFECTIVE_DATE

        // DateTime.Compare(y.EFFECTIVE_DATE, x.EFFECTIVE_DATE)
        var body = Expression.Call( typeof( DateTime ).GetMethod( "DateTime.Compare" ), yDate, xDate );

        // (x, y) => DateTime.Compare(y.EFFECTIVE_DATE, x.EFFECTIVE_DATE)
        var expr = Expression.Lambda<Func<AuthorizePayments, AuthorizePayments, int >>( body, paramX, paramY );

return expr; }

I know the variables are wrong at this time. The sort expression will not work for the sort method.

Am i just way off base for what I am trying to do?

Sorry for my ignorance and thanks for the help.

Answer 1

Well, you can "statically" create it with the compiler's help using a lambda expression:

Expression<Func<Foo, Foo, int>> expr =
            (x, y) => DateTime.Compare(y.EFFECTIVE_DATE, x.EFFECTIVE_DATE);

If you'd like to do it "by hand":

var paramX = Expression.Parameter(typeof(Foo), "x"); // x
var paramY = Expression.Parameter(typeof(Foo), "y"); // y

var xDate = Expression.Property(paramX, "EFFECTIVE_DATE"); // x.EFFECTIVE_DATE
var yDate = Expression.Property(paramY, "EFFECTIVE_DATE"); // y.EFFECTIVE_DATE

// DateTime.Compare(y.EFFECTIVE_DATE, x.EFFECTIVE_DATE)
var body = Expression.Call(typeof(DateTime).GetMethod("Compare"), yDate, xDate);

// (x, y) => DateTime.Compare(y.EFFECTIVE_DATE, x.EFFECTIVE_DATE)
var expr = Expression.Lambda<Func<Foo, Foo, int>>(body, paramX, paramY);