How do I create an expression tree to represent 'String.Contains("term")' in C#?

Question

I am just getting started with expression trees so I hope this makes sense. I am trying to create an expression tree to represent:

t => t.SomeProperty.Contains("stringValue");

So far I have got:

    private static Expression.Lambda<Func<string, bool>> GetContainsExpression<T>(string propertyName, string propertyValue)
    {
        var parameterExp = Expression.Parameter(typeof(T), "type");
        var propertyExp = Expression.Property(parameter, propertyName);
        var containsMethodExp = Expression.*SomeMemberReferenceFunction*("Contains", propertyExp) //this is where I got lost, obviously :)
        ...
        return Expression.Lambda<Func<string, bool>>(containsMethodExp, parameterExp); //then something like this
    }

I just don't know how to reference the String.Contains() method.

Help appreciated.

Cladophyll

Answer 1

Something like:

class Foo
{
    public string Bar { get; set; }
}
static void Main()
{
    var lambda = GetExpression<Foo>("Bar", "abc");
    Foo foo = new Foo { Bar = "aabca" };
    bool test = lambda.Compile()(foo);
}
static Expression<Func<T, bool>> GetExpression<T>(string propertyName, string propertyValue)
{
    var parameterExp = Expression.Parameter(typeof(T), "type");
    var propertyExp = Expression.Property(parameterExp, propertyName);
    MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    var someValue = Expression.Constant(propertyValue, typeof(string));
    var containsMethodExp = Expression.Call(propertyExp, method, someValue);

    return Expression.Lambda<Func<T, bool>>(containsMethodExp, parameterExp);
}

You might find this helpful.

Mehitable

What if I want to call does not startWith "substring" ? – Metternich

@AhmD Expression.Not around the Expression.Call, and change the GetMethod to StartsWith? – Mehitable

If you are using this code to provide an expression to an ORM like linq2db, you need to hoist the propertyValue into a class to have it turned into a SQL parameter. Otherwise you might have a SQL-injection vulnerability if the user is supplying the value from the UI. Something like var someValue = Expression.Property(Expression.Constant(new {Value = propertyValue}), "Value"); – Munroe

@Steve any Expression-based ORM that doesn't correctly either escape or parameterize a ConstantExpression is hopelessly broken. This by itself will not risk a SQL injection vulnerability. If you use hopelessly broken tools, then you're already at risk of ... anything. Do you have an actual example where this applies? This sounds like "FUD". – Mehitable

@MarcGravell Found this behaviour in linq2db when connecting to a Firebird database. I've added an issue to their repo to see what the authors think (github.com/linq2db/linq2db/issues/296) – Munroe

@Steve but do you have an example where it doesn't actually escape. It can be valid not to use parameters.... if the orm correctly escapes the input. Personally, I'd use a parameter for simplicity, but that is a design choice. – Mehitable

@MarcGravell You're right. linq2db does escape the text correctly before including it in the SQL. I was stuck on the thought that parameters were necessary. Many thanks for your comments to correct that! Have closed the linq2db issue I created. – Munroe

Answer 2

To perform a search like:

ef.Entities.Where(entity => arr.Contains(entity.Name)).ToArray();

which the trace string will be:

SELECT .... From Entities ... Where Name In ("abc", "def", "qaz")

I use the method I created below:

ef.Entities.Where(ContainsPredicate<Entity, string>(arr, "Name")).ToArray();

public Expression<Func<TEntity, bool>> ContainsPredicate<TEntity, T>(T[] arr, string fieldname) where TEntity : class {
  ParameterExpression entity = Expression.Parameter(typeof(TEntity), "entity");
  MemberExpression member = Expression.Property(entity, fieldname);

  var containsMethods = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public)
  .Where(m => m.Name == "Contains");
  MethodInfo method = null;
  foreach (var m in containsMethods) {
    if (m.GetParameters().Count() == 2) {
      method = m;
      break;
    }
  }
  method = method.MakeGenericMethod(member.Type);
  var exprContains = Expression.Call(method, new Expression[] { Expression.Constant(arr), member });
  return Expression.Lambda<Func<TEntity, bool>>(exprContains, entity);
}

Socratic

Answer 3

How about this:

Expression<Func<string, string, bool>> expFunc = (name, value) => name.Contains(value);

In the client code:

    bool result = expFunc.Compile()("FooBar", "Foo");   //result true
    result = expFunc.Compile()("FooBar", "Boo");        //result false

Heteropterous

Answer 4

Here is how to create an expression tree of string.Contains.

var method = typeof(Enumerable)
    .GetRuntimeMethods()
    .Single(m => m.Name == nameof(Enumerable.Contains) && m.GetParameters().Length == 2);
var containsMethod = method.MakeGenericMethod(typeof(string));
var doesContain = Expression
.Call(containsMethod, Expression.Constant(criteria.ToArray()),
 Expression.Property(p, "MyParam"));

Actual usage at https://raw.githubusercontent.com/xavierjohn/Its.Cqrs/e44797ef6f47424a1b145d69889bf940b5581eb8/Domain.Sql/CatchupEventFilter.cs

Rickrack