Question

I want to build an expression for a Where clause

Expression<Func<T, bool>>

on a method that accepts a parameter of type

Expression<Func<T, DateTime>>

I want to manipulate my DateTime property to compare its internal properties like to add compare clauses to day, month and year properties.

My method is like the following:

public static class MyUtils
{
    public static Expression<Func<T, bool>> GetTime<T>(this Expression<Func<T, bool>> pExp, Expression<Func<T, DateTime>> MyProperty, int day, int month, int year)
    {
    }
}

I’d like to use it like this:

Expression<Func<MyClass, bool>> vExpression = p => false;
vExpression = vExpression.GetTime<MyClass>(c => c.MyDate, 21, 12, 2012);

So at the end of the method I have some comparisons between my “MyDate” property and the individual date values.

Can this be done?

Answer 1

Perhaps something like this:

static Expression<Func<T, bool>> GetTime<T>(
    Expression<Func<T, DateTime>> expression, 
    DateTime compare
)
{
    var comparison = Expression.Equal(expression.Body, Expression.Constant(compare));
    return Expression.Lambda<Func<T, bool>>(comparison, expression.Parameters);
}

You can then do:

Expression<Func<MyClass, bool>> vExpression = GetTime<MyClass>(
    c => c.MyDate, 
    new DateTime(2012, 12, 21)
);

EDIT

The above code shows how you can create a new expression tree based on the old one. You can create your new expression tree using any allowed constructs. Here is a slightly more complicated example:

static Expression<Func<T, bool>> GetTime<T>(
    Expression<Func<T, DateTime>> expression, 
    DateTime compare
)
{
    var comparison = Expression.AndAlso(
        Expression.AndAlso(
            Expression.GreaterThanOrEqual(
                Expression.Property(expression.Body, "Year"),
                Expression.Constant(compare.Year)
            ),

            Expression.GreaterThanOrEqual(
                Expression.Property(expression.Body, "Month"),
                Expression.Constant(compare.Month)
            )
        ),

        Expression.GreaterThanOrEqual(
            Expression.Property(expression.Body, "Day"),
            Expression.Constant(compare.Day)
        )
    );

    return Expression.Lambda<Func<T, bool>>(comparison, expression.Parameters);
}