使用！DateTime.HasValue创建表达式树

Question

I have a very large expression tree builder that has been built and added to regularly as new functionality was needed. It has been working perfectly for all types including Nullable<> types. Several of my Nullable<> types are DateTime as those columns in my database allow nulls.

Now, I need to adjust the method to be able to filter based on a DateTime having ANY value. In SQL terms:

WHERE date_Column IS NOT NULL

In LINQ terms:

.Where(s => !s.date_column.HasValue)

For the life of me, I cannot figure out how to add this to my expression tree. I can add any operator to the MyFilter class that I like, so if a new case will help, that is no problem.

Below is the builder cut down to just the DateTime portion for readability.

public class MyFilter
{
    public string FieldName { get; set; }
    public string FieldValue { get; set; }
    public string Operator { get; set; }
}

private Expression<Func<MyDTO, bool>> CreateLambda(MyFilter myFilter)
{
    ParameterExpression parameter = Expression.Parameter(typeof(MyDTO), "m");
    Expression property = Expression.Property(parameter, myFilter.FieldName);
    Expression target = null;
    Expression exp = null;
    PropertyInfo pi = null;
    MethodInfo mi = null;

    var switchType = property.Type.ToString();

    switch (switchType)
    {
        case "System.DateTime":
            target = (myFilter.FieldValue == "null") ?
                Expression.Constant(null, property.Type) :
                Expression.Constant(Convert.ToDateTime(myFilter.FieldValue));
            switch (myFilter.Operator)
            {
                case "eq":
                    exp = Expression.Equal(property, Expression.Convert(target, property.Type));
                    break;
                case "ne":
                    exp = Expression.NotEqual(property, Expression.Convert(target, property.Type));
                    break;
                case "ge":
                    exp = Expression.GreaterThanOrEqual(property, Expression.Convert(target, property.Type));
                    break;
                case "gt":
                    exp = Expression.GreaterThan(property, Expression.Convert(target, property.Type));
                    break;
                case "le":
                    exp = Expression.LessThanOrEqual(property, Expression.Convert(target, property.Type));
                    break;
                case "lt":
                    exp = Expression.LessThan(property, Expression.Convert(target, property.Type));
                    break;
            }
            break;
    }
    Expression<Func<MyDTO, bool>> lambda = Expression.Lambda<Func<MyDTO, bool>>(exp, parameter);
    return lambda;
}

Answer 1

Filip Cordas was correct and both his statements produce the same correct result. I have no idea why Expression.NotEquals was not working for me yesterday, but the code above does work as is.

However, since my question was about how to use the HasValue property of a Nullable, I appreciated Filip's second answer very much. Also, now that I see it, it is humbling that I could not get it myself.

The code I implemented to use the HasValue within my environment looks like the following:

public class MyFilter
{
    public string FieldName { get; set; }
    public string FieldValue { get; set; }
    public string Operator { get; set; }
}

private Expression<Func<MyDTO, bool>> CreateLambda(MyFilter myFilter)
{
    ParameterExpression parameter = Expression.Parameter(typeof(MyDTO), "m");
    Expression property = Expression.Property(parameter, myFilter.FieldName);
    Expression target = null;
    Expression exp = null;
    PropertyInfo pi = null;
    MethodInfo mi = null;

    var switchType = property.Type.ToString();

    switch (switchType)
    {
        case "System.DateTime":
            target = (myFilter.FieldValue == "null") ?
                Expression.Constant(null, property.Type) :
                Expression.Constant(Convert.ToDateTime(myFilter.FieldValue));
            switch (myFilter.Operator)
            {
                case "eq":
                    exp = Expression.Equal(property, Expression.Convert(target, property.Type));
                    break;
                case "ne":
                    if (myFilter.FieldValue == "null")
                    {
                        exp = Expression.Property(property, "HasValue");
                    }
                    else
                    {
                        exp = Expression.NotEqual(property, Expression.Convert(target, property.Type));
                    }
                    break;
                case "ge":
                    exp = Expression.GreaterThanOrEqual(property, Expression.Convert(target, property.Type));
                    break;
                case "gt":
                    exp = Expression.GreaterThan(property, Expression.Convert(target, property.Type));
                    break;
                case "le":
                    exp = Expression.LessThanOrEqual(property, Expression.Convert(target, property.Type));
                    break;
                case "lt":
                    exp = Expression.LessThan(property, Expression.Convert(target, property.Type));
                    break;
            }
            break;
    }
    Expression<Func<MyDTO, bool>> lambda = Expression.Lambda<Func<MyDTO, bool>>(exp, parameter);
    return lambda;
}