C# Property Pattern

The property pattern is a pattern matching construct in C# that enables you to match an expression’s properties or fields against nested patterns. It evaluates to true if the input expression is non-null and every specified property or field matches its corresponding pattern.

Syntax

The property pattern is defined using curly braces { } containing a comma-separated list of property names, a colon :, and the nested pattern to match against that property.

{ Property1: <Pattern>, Property2: <Pattern> }

It is most frequently combined with a type pattern to ensure the object is of a specific type before evaluating its properties:

<Type> { Property1: <Pattern> }

Core Mechanics

Implicit Null Checking A property pattern inherently guarantees that the input is not null. If the input expression evaluates to null, the pattern immediately returns false without throwing a NullReferenceException. Consequently, an empty property pattern is commonly used as a concise non-null check.

// Evaluates to true if obj is not null
bool isNotNull = obj is { };

Constant and Relational Matching Properties can be matched against constant values or evaluated using relational patterns (<, >, <=, >=).

// Matches if obj is a Person, Age is 30 or greater, and Status is "Active"
bool match = obj is Person { Age: >= 30, Status: "Active" };

Variable Capture (Declaration Pattern) You can extract property values directly into local variables during the match by combining the property pattern with a declaration pattern.

// If obj is a Person, extracts the Name property into the 'personName' variable
if (obj is Person { Name: string personName })
{
    Console.WriteLine(personName);
}

// Using 'var' for implicit typing during capture
bool match = obj is Person { Age: var extractedAge };

Nested Property Patterns Because the right side of the colon accepts any valid C# pattern, property patterns can be nested recursively to evaluate complex object graphs.

// Matches if the Person's Address property has a City property equal to "Seattle"
bool match = obj is Person { Address: { City: "Seattle" } };

Extended Property Patterns (C# 10+) Starting in C# 10, you can reference nested properties directly using dot notation, eliminating the need for nested curly braces. This provides a more concise syntax for deep property evaluation.

// Functionally identical to the nested example above
bool match = obj is Person { Address.City: "Seattle" };

Logical Combinators Property patterns fully support logical combinators (and, or, not) within the nested pattern evaluation.

// Matches if Age is between 18 and 65, and Name is NOT null
bool match = obj is Person { Age: >= 18 and <= 65, Name: not null };

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​Syntax

​Core Mechanics

Syntax

Core Mechanics