Custom validation messages

Every validator attribute inherits a Message property from ValidatorAttribute. Set it to replace that validator’s default message. The value is a template: tokens wrapped in braces are substituted before the error is recorded.

[Validate]
public sealed partial record Query : IValidationTarget<Query>
{
	[GreaterThan(0, Message = "'{PropertyName}' must be a positive id, but was {PropertyValue}.")]
	public required int Id { get; init; }
}

Available tokens

Xxx is the validator’s ValidateProperty parameter name in PascalCase. GreaterThan takes a parameter called comparison, so its value is {ComparisonValue} and the name of a referenced member is {ComparisonName}. Length takes minLength and maxLength, giving {MinLengthValue} and {MaxLengthValue}. The built-in validators reference lists the parameter names for each attribute.

[Validate]
public sealed partial record Query : IValidationTarget<Query>
{
	public required int Minimum { get; init; }

	[GreaterThan(nameof(Minimum), Message = "'{PropertyName}' must exceed '{ComparisonName}' ({ComparisonValue}).")]
	public required int Maximum { get; init; }
}

With Minimum of 10, that renders as 'Maximum' must exceed 'Minimum' (10).

Format specifiers

A token may carry a standard .NET format string after a colon, which is applied via string.Format:

[Validate]
public sealed partial record Query : IValidationTarget<Query>
{
	[LessThan(0.0d, Message = "{PropertyValue:N2}")]
	public required double Value { get; init; }
}

A Value of 1.2345 renders 1.23.

Unknown tokens are left alone

An unrecognised token is emitted verbatim rather than throwing or being blanked out. A message of "{Invalid}" produces the literal error message {Invalid}. This is convenient when a message legitimately contains braces, but it also means a typo’d token is silent — check your rendered messages.

Display names

{PropertyName} renders a display name, not the C# identifier. The default is the identifier with a space inserted before each capital letter that follows a non-capital, so StringValue becomes String Value and Id stays Id. Collection indexes are appended: String Property[0].

Override it with [Description] from System.ComponentModel:

using System.ComponentModel;

[Validate]
public sealed partial record Query : IValidationTarget<Query>
{
	[Description("Customer number")]
	[GreaterThan(0)]
	public required int CustomerNumber { get; init; }
}

That renders 'Customer number' must be greater than '0'.

[Description] also applies to members referenced by nameof(...), so {ComparisonName} picks up the description of the member it points at.

How referenced members are named

For attribute-based validation, {XxxName} resolves as follows:

• Property or field — its [Description], else its humanized name.
• Method — the method name followed by (), for example AllDigitsRegex().
• Literal argument — the empty string.

Inside an AdditionalValidations method, the name is derived from the expression you wrote, so richer shapes are supported: nested member chains render as Parent.Child, array and indexer accesses as Parent[0] or Parent[123], and parameterless method calls as Method() or Parent.Method().

Changing the defaults globally

To change a validator’s message everywhere rather than per-property, replace the message catalogue instead — see Localization.