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# NSExpressionDescription

An object that describes an expression to include with a fetch request.

```
class NSExpressionDescription
```

## Overview

An expression description describes a value that a fetch request returns, which doesn’t appear as an attribute or relationship on an entity. For example, expressions can aggregate data, or  transform an attribute’s value. You add expression descriptions to a fetch request using the [`propertiesToFetch`](/documentation/CoreData/NSFetchRequest/propertiesToFetch) method.

> Important:
> Don’t add expression descriptions to the ``doc://com.apple.coredata/documentation/CoreData/NSEntityDescription/properties`` array of ``doc://com.apple.coredata/documentation/CoreData/NSEntityDescription``.

## Topics

### Configuring the Expression Description

[`expression`](/documentation/CoreData/NSExpressionDescription/expression)

The expression to evaluate.

[`resultType`](/documentation/CoreData/NSExpressionDescription/resultType)

The attribute type of the expression’s result.

[`expressionResultType`](/documentation/CoreData/NSExpressionDescription/expressionResultType)

The attribute type of the expression’s result.

### Deprecated

[Deprecated Symbols](/documentation/CoreData/nsexpressiondescription-deprecated-symbols)

Review unsupported symbols and their replacements.

## Relationships

### Conforms To

[`NSCopying`](/documentation/Foundation/NSCopying)

[`NSCoding`](/documentation/Foundation/NSCoding)

[`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol)

[`Hashable`](/documentation/Swift/Hashable)

[`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible)

[`CVarArg`](/documentation/Swift/CVarArg)

[`Equatable`](/documentation/Swift/Equatable)

[`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible)

### Inherits From

[`NSPropertyDescription`](/documentation/CoreData/NSPropertyDescription)

---

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