| title | Key (Visual Basic) | |||
|---|---|---|---|---|
| ms.date | 07/20/2015 | |||
| ms.prod | .net | |||
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| ms.topic | article | |||
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| ms.assetid | 7697a928-7d14-4430-a72a-c9e96e8d6c11 | |||
| caps.latest.revision | 22 | |||
| author | dotnet-bot | |||
| ms.author | dotnetcontent |
The Key keyword enables you to specify behavior for properties of anonymous types. Only properties you designate as key properties participate in tests of equality between anonymous type instances, or calculation of hash code values. The values of key properties cannot be changed.
You designate a property of an anonymous type as a key property by placing the keyword Key in front of its declaration in the initialization list. In the following example, Airline and FlightNo are key properties, but Gate is not.
[!code-vbVbVbalrAnonymousTypes#26]
When a new anonymous type is created, it inherits directly from xref:System.Object. The compiler overrides three inherited members: xref:System.Object.Equals%2A, xref:System.Object.GetHashCode%2A, and xref:System.Object.ToString%2A. The override code that is produced for xref:System.Object.Equals%2A and xref:System.Object.GetHashCode%2A is based on key properties. If there are no key properties in the type, xref:System.Object.GetHashCode%2A and xref:System.Object.Equals%2A are not overridden.
Two anonymous type instances are equal if they are instances of the same type and if the values of their key properties are equal. In the following examples, flight2 is equal to flight1 from the previous example because they are instances of the same anonymous type and they have matching values for their key properties. However, flight3 is not equal to flight1 because it has a different value for a key property, FlightNo. Instance flight4 is not the same type as flight1 because they designate different properties as key properties.
[!code-vbVbVbalrAnonymousTypes#27]
If two instances are declared with only non-key properties, identical in name, type, order, and value, the two instances are not equal. An instance without key properties is equal only to itself.
For more information about the conditions under which two anonymous type instances are instances of the same anonymous type, see Anonymous Types.
Like xref:System.Object.Equals%2A, the hash function that is defined in xref:System.Object.GetHashCode%2A for an anonymous type is based on the key properties of the type. The following examples show the interaction between key properties and hash code values.
Instances of an anonymous type that have the same values for all key properties have the same hash code value, even if non-key properties do not have matching values. The following statement returns True.
[!code-vbVbVbalrAnonymousTypes#37]
Instances of an anonymous type that have different values for one or more key properties have different hash code values. The following statement returns False.
[!code-vbVbVbalrAnonymousTypes#38]
Instances of anonymous types that designate different properties as key properties are not instances of the same type. They have different hash code values even when the names and values of all properties are the same. The following statement returns False.
[!code-vbVbVbalrAnonymousTypes#39]
The values of key properties cannot be changed. For example, in flight1 in the earlier examples, the Airline and FlightNo fields are read-only, but Gate can be changed.
[!code-vbVbVbalrAnonymousTypes#28]
Anonymous Type Definition
How to: Infer Property Names and Types in Anonymous Type Declarations
Anonymous Types