How to create linq WHERE IN statement without using Contains()

Question

I have a situation where I am using a linq provider that does not support the .Contains method to generate a WHERE IN clause in the query. I am looking for a way to generate the (Value = X OR Value = Y OR Value = Z) statement dynamically from the list of items to match. I haven't found a good example of building expression trees to do this.

Normally I would query this way:

var names = new string[] { "name1", "name2", "name3" }
var matches = query.Where(x => names.Contains(x.Name));

So far the closest thing I could find was to use the Dynamic Linq Library and build a string to be interpreted but it feels a bit too hacky.

Welcome to StackOverflow, It's better to show what you have tried so far and provide your question with some of your code. — Hossein Narimani Rad, May 20 '15 at 13:43
What would your LINQ statement be if you were to use the .Contains method? — Taegost, May 20 '15 at 13:43
There is a PredicateBuilder [here](http://www.albahari.com/nutshell/predicatebuilder.aspx). Note that it requires you append a `AsExpandable()` to your query. — xanatos, May 20 '15 at 13:44
If you really need this, [`PredicateBuilder`](http://www.albahari.com/nutshell/predicatebuilder.aspx) is a good starting point... — Jon Skeet, May 20 '15 at 13:44
Or I had written a small implementatin of it here: http://stackoverflow.com/a/22407189/613130 — xanatos, May 20 '15 at 13:46

score 0 · Answer 1 · answered May 20 '15 at 14:08

You don't even need an external library for something like this:

var names = new string[] { "name1", "name2", "name3" };

// Where MyClass is the type of your class
ParameterExpression par = Expression.Parameter(typeof(MyClass));
MemberExpression prop = Expression.Property(par, "Name");

Expression expression = null;

foreach (string name in names)
{
    Expression expression2 = Expression.Equal(prop, Expression.Constant(name));

    if (expression == null)
    {
        expression = expression2;
    }
    else
    {
        expression = Expression.OrElse(expression, expression2);
    }
}

var query = ...; // Your query

if (expression != null)
{
    // Where MyClass is the type of your class
    var lambda = Expression.Lambda<Func<MyClass, bool>>(expression, par);
    query = query.Where(lambda);
}

You can build a concatenation of Expression.OrElse, with the comparisons between the property Name and one of the strings.

In this particular case (3 strings), the resulting Expression, when looked from the debugger, is:

(((Param_0.Name == "name1") OrElse (Param_0.Name == "name2")) OrElse (Param_0.Name == "name3"))

score 0 · Answer 2 · answered May 20 '15 at 16:07

Just improving the answer by xanatos:

var names = new string[] { "name1", "name2", "name3" };

var query = ...; // Your query

if (names.Any())
{
    // Where MyClass is the type of your class
    ParameterExpression par = Expression.Parameter(typeof(MyClass));
    MemberExpression prop = Expression.Property(par, "Name");

    var expression=names
        .Select(v => Expression.Equal(prop, Expression.Constant(v)))
        .Aggregate(Expression.OrElse);

    // Where MyClass is the type of your class
    var lambda = Expression.Lambda<Func<MyClass, bool>>(expression, par);
    query = query.Where(lambda);
}

How to create linq WHERE IN statement without using Contains()

2 Answers2