Making generic expression generating method in C#

Question

I am constructing an IQueryable query using several methods. The methods are somehow complex, but the problem I want to solve can be extracted and simplified as follows. I have two methods

private Expression<Func<T, bool>> CreateExpressionA(string ValueA)
{
  return a => a.PropertyA.ToLower() == ValueA;
}

private Expression<Func<T, bool>> CreateExpressionB(string ValueB)
{
  return a => a.PropertyB.ToLower() == ValueB;
}

and what I would rather have is this:

private Expression<Func<T, bool>> CreateExpression(??? Selector, string Value)
{
  return a => a.Selector.ToLower() == Value;
}

or a similar approach that would allow me to avoid having two same methods with the only difference being in what property of an object is being used there.

Is it possible to do this in some elegant way?

It surely is (as I see in the answers), I just did not know how exactly to write it. But I like the non-reflection answer even more. — Wapac
– Wapac, Commented Nov 14, 2016 at 14:32

Thorkil Holm-Jacobsen · Accepted Answer · 2016-11-14 14:11:02Z

4

You can pass in a selector Func that returns a string property:

private Expression<Func<T, bool>> CreateExpression<T>(Func<T, string> selector, string value)
{
    return a => selector(a).ToLower() == value;
}

Usage:

CreateExpression<MyType>(x => x.PropertyA, "thevalue");

edited Nov 14, 2016 at 14:11

answered Nov 14, 2016 at 14:05

Thorkil Holm-Jacobsen

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1 Comment

Wapac Over a year ago

I love this solution. This is exactly what I was looking for, and it works well. Thanks!

Codor · Accepted Answer · 2016-11-14 14:04:38Z

1

You could use reflection, more precisely the class PropertyInfo as an argument, but the implementation would be more involved. The method could be implemented as follows.

private Expression<Func<T, bool>> CreateExpression(PropertyInfo iInfo, string Value)
{
    return a => (iInfo.GetPropertyValue(a) as string).ToLower() == ValueB;
}

However, note that this will work only if the type of the property is string, otherwise an additional type parameter could be used.

answered Nov 14, 2016 at 14:04

Codor

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Comments

Robba · Accepted Answer · 2016-11-14 14:05:54Z

0

What you can do is create the Expression from scratch:

private Expression<Func<T, bool>> CreateExpression(string propertyName, string value) {
    ParameterExpression param = Expression.Parameter(typeof(T));
    MemberExpression property = Expression.Property(param, propertyName);
    var valExpr = Expression.Constant(value);
    var body = Expression.Equal(property, valExpr);
    return Expression.Lambda<Func<T, bool>>(body, param);
}

Call with:

var expression = CreateExpression<TypeOfA>("PropertyA", "ValueForPropertyA");

It's a bit off the top of my head, but I think this should at least get you started. Let me know if you need help.

answered Nov 14, 2016 at 14:05

Robba

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