I recently was working with some dynamic Search Expressions and ran into a bit of an issue when I was attempting to search for multiple Date/Times.

My SearchExpression constructors resemble the following (some omitted for brevity):

public SearchExpression(string propertyName, 
                        ComparisonOperator comparisonOperator, 
                        object value) {...}

and

public SearchExpression(string propertyName, 
                        ComparisonOperator comparisonOperator, 
                        object[] values, 
                        BooleanOperator innerBooleanOperator) {...}

Both of these work as they should when passed the following :

• Strings
• Numbers (or any numerical data)

However, when it comes to passing DateTime information, I began to encounter some problems.

I have all of my data seperated when it comes in based on the type (DateTime, Numerical or String) and ch开发者_StackOverflow中文版eck for validity, they are then stored into array to pass to the proper expressions, like so:

object[] stringParameters;
object[] numericalParameters;
DateTime[] dateParameters;

All of these work when I pass in any of the above, with the exception of an array of DateTime parameters.

So my question, which leaves me feeling a bit foolish, is why can I pass a single DateTime value as an object, but am unable to pass an DateTime[] as an object[] .

Example:

DateTime[] dateParameters;

//This works fine
new SearchExpression("DateTime", ComparisonOperator.Equals, dateParameters[0]);

//This fails to work
new SearchExpression("DateTime", ComparisonOperator.Equals, dateParameters,
                     BooleanOperator.Or);

Why can I pass a single DateTime value as an object?

There is an implicit boxing conversion available from DateTime (a value-type) to object.

But am unable to pass an DateTime[] as an object[]?

There is no implicit conversion available from a DateTime[] to object[]. Although arrays in C# exhibit covariance in general (for example, there is an implicit conversion from string[] to object[]), this does not extend to arrays of value-types - covariant conversions are currently required to be identity-preserving conversions, with good reason.

This is inherently a problem of Covariance which arrays do not support arrays do not support for value types.

Eric Lippert has a very good post on that here.

Solution

Change method signature to IEnumerable<object>.

you can pass/cast DateTime[] as object as well, but not as object[] because it needs casting/converting for each item in the array. compiler will complain about the latter.

you can write Static Extension/Linq to convert DateTime[] into object[].

when function is expecting object[], it is not expecting array of objects.

when function is expecting object, you can pass DateTime, because DateTime being of type "struct" inherits from object. hope it helps :)