custom-validation-checkvalue.md

Custom Validation with CheckValue

CheckValue lets you enforce invariants and normalize values in generated identifiers.

Scalar IDs

For [StringId]:

private static string CheckValue(string value)

For [GuidId]:

private static Guid CheckValue(Guid value)

Example:

[StringId]
public sealed partial class InvoiceNumber
{
    private static string CheckValue(string value)
    {
        if (string.IsNullOrWhiteSpace(value))
            throw new ArgumentException("Invoice number is required", nameof(value));

        return value.Trim().ToUpperInvariant();
    }
}

Combined IDs

For [CombinedId], define parameters matching constructor components and return a tuple of the same shape.

private static (Guid TenantId, string UserCode) CheckValue(Guid tenantId, string userCode)

Example:

[CombinedId(typeof(Guid), "TenantId", typeof(string), "UserCode")]
public sealed partial class TenantUserId
{
    private static (Guid TenantId, string UserCode) CheckValue(Guid tenantId, string userCode)
    {
        if (tenantId == Guid.Empty)
            throw new ArgumentException("TenantId cannot be empty", nameof(tenantId));

        if (string.IsNullOrWhiteSpace(userCode))
            throw new ArgumentException("UserCode is required", nameof(userCode));

        return (tenantId, userCode.Trim());
    }
}

Behavior notes

• CheckValue is called by the generated constructor.
• Throwing exceptions prevents invalid IDs from being created.
• Returned values are used to initialize generated properties.

See also private constructors and factories.