Generic LINQ query predicate?
Solution 1
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: Another set of posts on this subject, using C# 4's Dynamic support: Part 1 and Part 2.
Solution 2
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
Solution 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);
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Comments
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Codewerks over 2 years
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?
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Chris Moschini about 11 yearsSame question but for Select clause: stackoverflow.com/questions/10376947/…
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Codewerks over 15 yearsI think the Dynamic LINQ answer is what I'm going to go with, but thanks for this, I will also try and work through this tomorrow.
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Codewerks over 15 yearsThanks! Very cool, I need to look through all of ScottGu's blog posts on this topic...
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Erik Forbes over 15 yearsUnfortunately I've only found the one - I thought he had a series but I guess I was wrong. There are other sources of information though.
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Thomas over 9 yearssorry to ask one question here. what is predicate and how it is related to linq?
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Erik Forbes over 9 yearsA predicate is an expression that evaluates to a bool. Predicates are used as arguments to Where clauses in LINQ expressions. In the example above,
CategoryID == 2 && UnitPrice > 3
is a predicate. -
AliReza Sabouri over 2 yearsI think Gridify is a better alternative. github.com/alirezanet/Gridify
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Erik Forbes over 2 years@AliReza I'll check it out. =)