c＃LINQ OrderBy，动态对象和空值

Question

I'm attempting to generate a dynamic expression that can order a list of ExpandoObjects. I don't know what will be in the expandoobjects (string, int, decimal, datetime, etc).

Unfortunately, it appears that if a null value is in the list when doing an orderby, an exception is thrown. I could remove the null values from my collection prior to sorting with a Where method, but I want to keep the null rows in the results returned.

What I was trying to do was generate an If Else statement, something like:

x => x.Item["Key"] != null ? x.Item["Key] : defaultvaluefortype

Here's my code snippet:

if (type == typeof (ExpandoObject))
        {
            arg = Expression.Parameter(typeof (T), "x");
            expr = arg;  //Get type of T
            var first = (IDictionary<string, object>) source.First();
            //Match the case of the string to the correct key value.
            var propval =
                first.Keys.First(x => String.Equals(x, prop, StringComparison.CurrentCultureIgnoreCase)); 
            var key = Expression.Constant(propval, typeof(string));
            ParameterExpression dictExpr = Expression.Parameter(typeof(IDictionary<string, object>));
            var indexer = dictExpr.Type.GetProperty("Item");
            var exprkeyed = Expression.Property(expr, indexer, key);
            //Generates x.Item["KeyString"] so I can access the object.
            expr = exprkeyed;
            var Null = Expression.Constant(null);
            expr = Expression.NotEqual(expr, Null);
            expr = Expression.Condition(expr, exprkeyed, Expression.Constant(false)); //what do I return as the else?
            type = typeof(Object);
            }

Unfortunately, if I try to set a default based on the key type, I get an exception that the return values from my if/else don't match (ie; one is system.object, one is system.datetime). I believe the default value for object is null as well, so that's not the best.

Is there a way to do this without using a where statement to remove the null entries first? Maybe something I could return on the else that's like a skip or a sortlow/high?

Thanks for your time.

Answer 1

You would return a Expression.Default(typeof(T)) . In your code you change

expr = Expression.Condition(expr, exprkeyed, Expression.Default(typeof(T)));