Linq query built in foreach loop always takes parameter value from last iteration
Solution 1
You're reusing the same variable (key
) in your lambda expression.
See my article on anonymous methods for more details, and there are a number of related SO questions too:
- LINQ to SQL bug (or very strange feature)...
- Local variables with delegates
- C# captured variable in a loop
- C# gotcha answer
- Building a LINQ query programmatically without local variables tricking me
The simple fix is to copy the variable first:
List<string> keys = FillKeys()
foreach (string key in keys){
string copy = key;
q = q.Where(c => c.Company.Name.Contains(copy));
}
Solution 2
Possibly a captured variable issue; try adding:
List<string> keys = FillKeys()
foreach (string key in keys){
string tmp = key;
q = q.Where(c => c.Company.Name.Contains(tmp));
}
Solution 3
it’s been fixed in C# 5.0, and the example above in C# 5.0 works but fails in earlier versions of C#.
But be careful, it does not concern a for loop
static void Main()
{
IEnumerable<char> query = "aaa bbb ccc";
string lettersToRemove = "ab";
Console.WriteLine("\nOK with foreach:");
foreach (var item in lettersToRemove)
{
query = query.Where(c => c != item);
}
foreach (char c in query) Console.Write(c);
//OK:
Console.WriteLine("\nOK with foreach and local temp variable:");
query = "aaa bbb ccc";
foreach (var item in lettersToRemove)
{
var tmp = item;
query = query.Where(c => c != tmp);
}
foreach (char c in query) Console.Write(c);
/*
An IndexOutOfRangeException is thrown because:
firstly compiler iterates the for loop treating i as an outsite declared variable
when the query is finnaly invoked the same variable of i is captured (lettersToRemove[i] equals 3) which generates IndexOutOfRangeException
The following program writes aaa ccc instead of writing ccc:
Each iteration gets the same variable="C", i (last one frome abc).
*/
//Console.WriteLine("\nNOK with for loop and without temp variable:");
//query = "aaa bbb ccc";
//for (int i = 0; i < lettersToRemove.Length; i++)
//{
// query = query.Where(c => c != lettersToRemove[i]);
//}
//foreach (char c in query) Console.Write(c);
/*
OK
The solution is to assign the iteration variable to a local variable scoped inside the loop
This causes the closure to capture a different variable on each iteration.
*/
Console.WriteLine("\nOK with for loop and with temp variable:");
query = "aaa bbb ccc";
for (int i = 0; i < lettersToRemove.Length; i++)
{
var tmp = lettersToRemove[i];
query = query.Where(c => c != tmp);
}
foreach (char c in query) Console.Write(c);
}
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Boris Callens
Senior .net programmer. Belgium(Antwerp) based. linked-in My real email is gmail.
Updated on April 17, 2022Comments
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Boris Callens about 2 years
I have a List containing several keywords. I foreach through them building my linq query with them like so (boiled down to remove the code noise):
List<string> keys = FillKeys() foreach (string key in keys){ q = q.Where(c => c.Company.Name.Contains(key)); }
When I now make my keys contain 2 keys that return results seperatly, but can never occure together (every item in q is either "xyz" or "123", never "123" AND "xyz"), I still get results. The resultset is then the same as the last string it got to.
I had a look at the linq query and it appears it creates the correct sql, but it replaces @p1 AND @p2 both by the same (last itterated) value.
What am I doing wrong?
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Boris Callens over 15 yearsHehe, found this one in the meantime: stackoverflow.com/questions/190227/… Easy (but earned) credits to you ;)
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Boris Callens over 15 yearsThanks. Basically same thing as skeet, but I can only assign one solution. Points though.
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Marc Gravell over 15 yearsNo problem. I find it reassuring that we gave the same reply ;-p