Generic LINQ query predicate?

Question

Not sure if this is possible or if I'm expressing correctly what I'm looking for, but I have the following piece of code in my library repeatedly and would like to practice some DRY. I have set of SQL Server tables that I'm querying based on a simple user-supplied search field ala Google. I'm using LINQ to compose the final query based on what's in the search string. I'm looking for a way to use generics and passed in lambda functions to create a reusable routine out of this:

string[] arrayOfQueryTerms = getsTheArray();

var somequery = from q in dataContext.MyTable
                select q;

if (arrayOfQueryTerms.Length == 1)
{
    somequery = somequery.Where<MyTableEntity>(
        e => e.FieldName.StartsWith(arrayOfQueryTerms[0]));
}
else
{
    foreach(string queryTerm in arrayOfQueryTerms)
    {
        if (!String.IsNullOrEmpty(queryTerm))
        {
            somequery = somequery 
                        .Where<MyTableEntity>(
                            e => e.FieldName.Contains(queryTerm));
        }
    }
}

I was hoping to create a generic method with signature that looks something like:

private IQueryable<T> getQuery(
    T MyTableEntity, string[] arrayOfQueryTerms, Func<T, bool> predicate)

I'm using the same search strategy across all my tables, so the only thing that really differs from usage to usage is the MyTable & MyTableEntity searched and the FieldName searched. Does this make sense? Is there a way with LINQ to dynamically pass in the name of the field to query in the where clause? Or can I pass in this as a predicate lambda?

e => e.FieldName.Contains(queryTerm)

I realize there a million and a half ways to do this in SQL, probably easier, but I'd love to keep everything in the LINQ family for this one. Also, I feel that generics should be handy for a problem like this. Any ideas?

Answer 1

What it sounds like is you want basically a conditional predicate builder..

I hope you can mold this into something you are looking for, good luck!

http://www.albahari.com/nutshell/predicatebuilder.aspx

Answer 2

It sounds like you're looking for Dynamic Linq. Take a look here. This allows you to pass strings as arguments to the query methods, like:

var query = dataSource.Where("CategoryID == 2 && UnitPrice > 3")
                      .OrderBy("SupplierID");

Edit: Found two really nice articles on Dynamic LINQ here and here.

Edit: Found another article on Dynamic LINQ regarding keeping your Dynamic LINQ queries safe from exploits similar to dynamic SQL queries. I feel this is important to share.

Edit: Another set of posts on this subject, using C# 4's Dynamic support: Part 1 and Part 2.

Answer 3

You might want to look at expression trees:

IQueryable<T> getQuery<T>(  T                         myTableEntity
                          , string[]                  arrayOfQueryTerms
                          , Expression<Func<T, bool>> predicate)
 { var fieldOrProperty = getMemberInfo(predicate);
   /* ... */
 }

MemberInfo getmemberInfo<T>(Expression<Func<T,bool> expr)
 { var memberExpr = expr as MemberExpression;
   if (memberExpr != null) return memberExpr.Member;
   throw new ArgumentException();
 }

var q = getQuery<FooTable>(foo, new[]{"Bar","Baz"}, x=>x.FieldName);

Answer 4

I recently had to do this same thing. You will need Dynamic Linq here is a way to keep this strongly typed.