C# Array.Contains () compilation error

Question

I'm trying to use the Array.Contains () method in C#, and for some reason it's failing to compile, eve though I believe that I'm using C# 4.0, and C# should support this in 3.0 and later.

if (! args.Contains ("-m"))
    Console.WriteLine ("You must provide a message for this commit.");

And I get this error:

Main.cs(42,15): error CS1061: 'System.Array' does not contain a definition for 'Contains' and no extension method 'Contains' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

I am compiling from the command line, with no options: "csc Main.exe".

Read the error message. It is telling you what is wrong. You are missing either an assembly reference or a using directive. — Eric Lippert
– Eric Lippert, Commented Apr 30, 2011 at 0:41

Blindy · Accepted Answer · 2011-04-29 23:22:16Z

82

You need to add using System.Linq; at the beginning of your program.

answered Apr 29, 2011 at 23:22

Blindy

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Comments

Jon · Accepted Answer · 2011-04-29 23:22:45Z

13

Did you forget using System.Linq?

By the way, if you can't use LINQ there are many other options such as Array.Exists.

answered Apr 29, 2011 at 23:22

Jon

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1 Comment

Blindy Over a year ago

Or Array.Contains, which does the same thing as he wants. But he's obviously using C# 3.5 so no reason why he couldn't use Linq.

CharithJ · Accepted Answer · 2011-04-29 23:25:39Z

9

If you dont' want to use linq try

((IList<string>)args).Contains("-m")

answered Apr 29, 2011 at 23:25

CharithJ

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Joakim M. H. · Accepted Answer · 2018-12-06 20:17:11Z

5

The answers saying to include System.Linq are spot on, however there is one other cause of this problem. If the type of the argument for .Contains() does not match the type of the array, you will get this error.

public class Product
{
    public long ProductID {get;set;}
}

public IEnumerable<Product> GetProductsByID(int[] prodIDs)
{
    using (var context = new MyDatabaseContext())
    {
        return context.Products.Where(product => prodIDs.Contains(product.ProductID)); // ['System.Array' does not contain a definition for 'Contains'] error because product.ProductID is long and prodIDs is an array of ints.
    }
}

answered Dec 6, 2018 at 20:17

Joakim M. H.

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1 Comment

Bas Over a year ago

This was exactly the problem I had. I kept looking at that using statement looking for typos. Confusing!

Yohan Dahmani · Accepted Answer · 2016-08-19 07:05:33Z

3

I had the same issue and I had

using System.Linq

It was because I was trying to compare string to int, but somehow it was saying

'System.Array' does not contain a definition for 'Contains'

answered Aug 19, 2016 at 7:05

Yohan Dahmani

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Comments

Alexei Levenkov · Accepted Answer · 2011-04-29 23:46:20Z

0

Make sure you are using correct version of CSC (csc /?) - you need 2010 version to compile for 4.0. You also may need to add additional libraries (/r option) for compile to succeed.

answered Apr 29, 2011 at 23:46

Alexei Levenkov

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