System.Collections.Generic.KeyNotFoundException

The System.Collections.Generic.Dictionary`2 means that the type is System.Collections.Generic.Dictionary, with two type arguments. So in this case it means that the type is System.Collections.Generic.Dictionary<TKey, TValue>, as we all know it.

By Patrick Hofman. Read the original answer on Stack Overflow.

You somehow convinced the C# compiler that your source code was written in code page 1251, the default system code page in Eastern Europe and Russia. That's usually caused by the text file missing the utf-8 BOM. Unclear how this happened, maybe you created the file with a text editor other than the one built into Visual Studio. Maybe it got mangled by source control, the ones with a Unix background tend to drop the BOM.

Open the source file in Visual Studio and ensure it still reads correctly. Then use File > Save As, click the arrow on the Save button, select "with encoding" and pick "Unicode (UTF-8 with signature)".

Also make sure that the default is still good. File > Advanced Save Options > change the Encoding if necessary. If you habitually use another text editor then you'll want configure it so it saves files with a BOM.

By Hans Passant. Read the original answer on Stack Overflow.

This's the way .Net makes classes' names. The initial declaration

 Dictionary<K, V>

will be turned into Dictionary'2 type name where '2 means two generic parameters:

 // Dictionary`2 - two generic parameters
 Console.WriteLine(typeof(Dictionary<int, string>).Name);

 // List`1 - one generic parameter
 Console.WriteLine(typeof(List<int>).Name);

Please compare:

 // IDictionary`2 - two generic parameters
 Console.WriteLine(typeof(IDictionary<int, string>).Name);

 // IDictionary - no generic parameters
 Console.WriteLine(typeof(System.Collections.IDictionary).Name);

By Dmitrii Bychenko. Read the original answer on Stack Overflow.

Let me try to answer all of your questions:

1. Is this expected behavior and is it documented somewhere?

Yes, it is documented in the C# 6.0 Language Specification under sections §7.6.11.2 Object initializers and §7.6.11.3 Collection initializers.

The syntax

var a =
    new Test
    {
        [1] = "foo"
        [2] = "bar"
    };

was actually newly introduced in C# 6.0 as an extension of the previous object initialization syntax to indexers. An object initializer used together with new (see object creation expression, §7.6.11) always translates to object instantiation and member access of the corresponding object (using a temporary variable), in this case:

var _a = new Test();
_a[1] = "foo";
_a[2] = "bar";
var a = _a;

The collection initializer goes similar besides that each element of the initializer is passed as an argument to the Add method of the newly created collection:

var list = new List<int> {1, 2};

becomes

var _list = new List<int>();
_list.Add(1);
_list.Add(2);
var list = _list;

An object initializer can also contain other object or collection initializers. The specification states for the case of collection initializers:

A member initializer that specifies a collection initializer after the equals sign is an initialization of an embedded collection. Instead of assigning a new collection to the target field, property or indexer, the elements given in the initializer are added to the collection referenced by the target.

So a sole collection initalizer used within an object initializer will not attempt to create a new collection instance. It will only try to add the elements to an exisiting collection, i.e. a collection that was already instantiated in the constructor of the parent object.

Writing

[1] = new List<string> { "str1", "str2", "str3" }

is actually a totally different case because this is an object creation expression which only contains an collection initializer, but isn't one.

2. Why is the syntax above allowed but the following syntax is not?

   List<string> list = { "test" };
   

Now, this is not a collection initializer anymore. A collection initalizer can only occur inside an object initializer or in an object creation expression. A sole { obj1, obj2 } next to an assignment is actually an array initializer (§12.6). The code does not compile since you can't assign an array to a List<string>.

3. Why is the following syntax not allowed then?

   var dict = new Dictionary<int, string[]> 
   {
       [1] = { "test1", "test2", "test3" },
       [2] = { "test4", "test5", "test6" }     
   };
   

It is not allowed because collection initalizers are only allowed to initialize collections, not array types (since only collections have an Add method).

By adjan. Read the original answer on Stack Overflow.