Change parameter from lambda function to lambda expression

Question

I have extension method:

public static IQueryable<TResult> WithFieldLike<TResult>(
   this IQueryable<TResult> query,
   Func<TResult, string> field,
   string value)
{
   Expression<Func<TResult, bool>> expr = 
       trans => field(trans).Contains(value);
   return query.Where(expr);
}

I need change parameter field to type: Expression>. Will be something like.

public static IQueryable<TResult> WithFieldLike<TResult>(
   this IQueryable<TResult> query,
   Expression<Func<TResult, string>> field,
   string value)
{
   Expression<Func<TResult, bool>> expr = ???
   return query.Where(expr);
}

The call of this method is:

var query7 = query.WithFieldLike(trans => trans.DeviceModelNumber, "ber_3");

How should I build the "expr" in this case? Please help.

Please add the language to the tags.

Svante
– Svante

2009-03-24 15:06:25 +00:00
Commented Mar 24, 2009 at 15:06 — Svante
– Svante, Commented Mar 24, 2009 at 15:06

Anton Tykhyy · Accepted Answer · 2009-03-24 16:03:05Z

6

Deconstruct field and create a new expression, something like this:

var expr = Expression.Lambda<Func<TResult, bool>> (
    Expression.Call (field.Body, typeof (string).GetMethod ("Contains"),
        Expression.Constant (value)), field.Parameters) ;

(edited as per Maxs's refinement in comments)

edited Mar 24, 2009 at 16:03

answered Mar 24, 2009 at 15:16

Anton Tykhyy

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3 Comments

Maxs Over a year ago

The point is that I can't build "Contains" expression. How to build "Equals" expression I know.

Maxs Over a year ago

The final result is: var expr1 = Expression.Lambda<Func<TResult, bool>>( Expression.Call(field.Body, typeof(string).GetMethod("Contains"), Expression.Constant(value)) , field.Parameters);

Anton Tykhyy Over a year ago

Oh, sheesh. I didn't see Expression.Lambda<>() in ildasm ^ω^

Marc Gravell · Accepted Answer · 2009-03-24 15:26:30Z

4

You'll need to use Expression.Invoke; something like (untested):

public static IQueryable<TResult> WithFieldLike<TResult>(
   this IQueryable<TResult> query,
   Expression<Func<TResult, string>> field,
   string value)
{
    var param = Expression.Parameter(typeof(TResult), "x");
    var expr = Expression.Lambda<Func<TResult, bool>>(
        Expression.Call(Expression.Invoke(field, param),
            "Contains", null, Expression.Constant(value)), param);

    return query.Where(expr);
}

(edit: fixed)

edited Mar 24, 2009 at 15:26

answered Mar 24, 2009 at 15:20

Marc Gravell

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2 Comments

Maxs Over a year ago

Thanks. This works too. But generates expression incompatible with ADO.NET Data Services call.

Marc Gravell Over a year ago

Gotta love the Law of Leaky Abstractions... it works with some but not all ;-p

Mark Brackett · Accepted Answer · 2009-03-24 20:09:00Z

1

Use Compile to get the lambda back out:

Expression<Func<TResult, bool>> expr = 
   trans => field.Compile()(trans).Contains(value);

Edit: Whoops - my air compiler failed me. After compiling, you get the delegate. But, you still need to call it to get the string to call Contains.

edited Mar 24, 2009 at 20:09

answered Mar 24, 2009 at 15:11

Mark Brackett

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2 Comments

Maxs Over a year ago

Will be compilation error: 'System.Func<TResult,string>' does not contain a definition for 'Contains' and no extension method 'Contains' accepting a first argument of type 'System.Func<TResult,string>' could be found (are you missing a using directive or an assembly reference?)

Mark Brackett Over a year ago

@Maxs: Yeah - I kind of screwed up the whole "converting it to another expression" part. Point stands though - use Compile to get the lambda, and use it as normal.

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