FDDB126: Key property references non-compile-time-constant value

Field	Value
Code	FDDB126
Severity	Error

Message

Property '{0}' uses expression-body or read-only auto-property syntax but its value is not a compile-time constant — use a string literal or a 'const' field instead

Description

A key property (annotated with [PartitionKey] or [SortKey]) uses expression-body (=>) or read-only auto-property ({ get; }) syntax, but its value is not a compile-time constant. References to static readonly fields, properties, or method calls cannot be resolved at compile time and will produce uncompilable generated code.

The source generator detects constant key values so it can skip property assignment in the FromDynamoDb() mapper (since the property has no setter). When the value cannot be resolved as a compile-time constant, the generator cannot safely generate code for this property.

Use a string literal (e.g., => "VALUE") or a const field (e.g., => MyConstants.Value where Value is const) instead.

Example

The following code triggers this diagnostic:

public static class StaticFields
{
    public static readonly string KeyValue = "PROFILE";  // readonly, NOT const
}

[DynamoDbTable("customers")]
public partial class Customer
{
    [PartitionKey]
    [DynamoDbAttribute("pk")]
    public string Pk { get; set; } = string.Empty;

    [SortKey]
    [DynamoDbAttribute("sk")]
    public string Sk => StaticFields.KeyValue;  // ❌ FDDB126: not a compile-time constant
}

Other patterns that trigger this diagnostic:

// Read-only auto-property with static readonly initializer
[SortKey]
[DynamoDbAttribute("sk")]
public string Sk { get; } = StaticFields.KeyValue;  // ❌ FDDB126

// Expression-body with method call
[SortKey]
[DynamoDbAttribute("sk")]
public string Sk => GetKey();  // ❌ FDDB126

// Expression-body with property access
[SortKey]
[DynamoDbAttribute("sk")]
public string Sk => Config.DefaultKey;  // ❌ FDDB126

Fix

Use a string literal or a const field reference:

// Option 1: String literal (simplest)
[SortKey]
[DynamoDbAttribute("sk")]
public string Sk => "PROFILE";  // ✅ Compile-time constant

// Option 2: Const field reference
public const string ProfileKey = "PROFILE";

[SortKey]
[DynamoDbAttribute("sk")]
public string Sk => ProfileKey;  // ✅ Const field resolves at compile time

// Option 3: External const field
public static class Keys
{
    public const string Profile = "PROFILE";
}

[SortKey]
[DynamoDbAttribute("sk")]
public string Sk => Keys.Profile;  // ✅ Const field resolves at compile time