Incorporate partial expression in LINQ/SQL query

Question

I have an expression that returns a DateTime column in a query.

Expression<Func<T, DateTime>> GetDateExpression;

GetDateExpression = td => td.DateShipped;

But now I want to use that expression to form a filter in a SQL query.

query = query.Where(td => GetDateExpression(td) >= startDate);

But that won't compile.

error CS1955: Non-invocable member 'DateFilter.GetDateExpression' cannot be used like a method.

Can anyone show me how to construct the where clause above using GetDateExpression?

Answer 1

You can write a method to construct a new GreaterThanOrEqual expression from two operands

The idea behind this is the following. You want to construct a new expression which looks like this:

td => td.DateShipped >= startDate.Value

using the body of your existing expression td => td.DateShipped, but important thing here is that td in the result expression and td in your GetDateExpression are different things, so if you just will write the greaterThanOrEqual expression without replacing you will get something like this:

td1 => td2.DateShipped >= startDate.Value

Therefore you need to replace td2 with td1 so that the expression looks like I wrote in the beginning. Therefore all the replacer does is replace each ParameterExpression it finds in the expression tree with our filterParam.

You can take a look at the following answers to read about this a bit more:

• Why we need replacer in the first place
• Implementation of ParameterReplacer used in my answer
• Great answer with different solutions for different cases

    Expression<Func<T, bool>> GreaterThanOrEqual(Expression<Func<T, DateTime?>> operand1, Expression<Func<T, DateTime?>> operand2)
    {
        var filterParam = Expression.Parameter(typeof(T));
        var greaterThanOrEqual = Expression.GreaterThanOrEqual(operand1.Body, operand2.Body);
        greaterThanOrEqual = (BinaryExpression) new ParameterReplacer(filterParam).Visit(greaterThanOrEqual);
        
        return Expression.Lambda<Func<T, bool>>(greaterThanOrEqual, filterParam);
    }
                                                                      
    internal class ParameterReplacer : ExpressionVisitor {
        private readonly ParameterExpression _parameter;

        protected override Expression VisitParameter(ParameterExpression node) {
            return base.VisitParameter(_parameter);
        }

        internal ParameterReplacer(ParameterExpression parameter) {
            _parameter = parameter;
        }
    }

Then use it like

    Expression<Func<T, DateTime?>> getDateExpression = m => m.Date;
    Expression<Func<MyClass, bool>> combined = GreaterThanOrEqual(getDateExpression, td => startDate.Value);
    
    query = query.Where(combined);