Pattern Matching

NOTE

Pattern matching support is available starting from version 6.x.

The source generator rewrites C# pattern-matching constructs into expression-tree-compatible ternary and binary expressions that EF Core can translate to SQL CASE expressions.

Switch Expressions

Relational and Constant Patterns

[Projectable]
public string GetGrade() => Score switch
{
    >= 90 => "A",
    >= 80 => "B",
    >= 70 => "C",
    _     => "F",
};

Generated expression:

(@this) => @this.Score >= 90 ? "A"
         : @this.Score >= 80 ? "B"
         : @this.Score >= 70 ? "C"
         : "F"

Which EF Core translates to:

SELECT CASE
    WHEN [e].[Score] >= 90 THEN N'A'
    WHEN [e].[Score] >= 80 THEN N'B'
    WHEN [e].[Score] >= 70 THEN N'C'
    ELSE N'F'
END

and / or Combined Patterns

[Projectable]
public string GetBand() => Score switch
{
    >= 90 and <= 100 => "Excellent",
    >= 70 and < 90   => "Good",
    _                => "Poor",
};

when Guards

[Projectable]
public string Classify() => Value switch
{
    4 when IsSpecial => "Special Four",
    4               => "Regular Four",
    _               => "Other",
};

Type Patterns

Type patterns in switch arms produce a cast and type-check expression:

[Projectable]
public static ItemData ToData(this Item item) =>
    item switch
    {
        GroupItem g    => new GroupData(g.Id, g.Name, g.Description),
        DocumentItem d => new DocumentData(d.Id, d.Name, d.Priority),
        _              => null!
    };

Discard / Default

The discard pattern (_) maps to the final else branch of the generated ternary chain.

---

is Patterns in Expression Bodies

Relational and / or

// Range check
[Projectable]
public bool IsInRange => Value is >= 1 and <= 100;
// → Value >= 1 && Value <= 100

// Alternative values
[Projectable]
public bool IsEdge => Value is 0 or 100;
// → Value == 0 || Value == 100

not null / not

[Projectable]
public bool HasName => Name is not null;
// → !(Name == null)

Property Patterns

[Projectable]
public static bool IsActiveAndPositive(this Entity entity) =>
    entity is { IsActive: true, Value: > 0 };
// → entity != null && entity.IsActive == true && entity.Value > 0

---

Supported Pattern Summary

Pattern	Context	Example
Constant	switch arm, is	1 => "one", Value is 42
Discard / default	switch arm	_ => "other"
Relational	switch arm, is	>= 90 => "A", Value is > 0
and combined	switch arm, is	>= 80 and < 90, Value is >= 1 and <= 100
or combined	switch arm, is	1 or 2 => "low", Value is 0 or > 100
not	is	Name is not null
when guard	switch arm	4 when Prop == 12 => …
Type	switch arm	GroupItem g => …
Property	is	entity is { IsActive: true }

---

Unsupported Patterns

The following patterns cannot be translated into expression trees and produce diagnostic EFP0007:

• Positional / deconstruct patterns: (0, 0) => …
• Variable designations outside switch arms: item is GroupItem g in an if condition
• List patterns: [1, 2, ..]
• var patterns

// ❌ EFP0007 — positional pattern not supported
[Projectable]
public bool IsOrigin(Point p) => p is (0, 0);

// ✅ Use a property pattern instead
[Projectable]
public bool IsOrigin(Point p) => p is { X: 0, Y: 0 };

See Diagnostics Reference for the full EFP0007 description.