dotnet/wpf · error · ArgumentException

The parameter must not be the default value.

Error message

The parameter must not be the default value.

What it means

Verify.IsNotDefault<T> validates that a value-type argument does not equal default(T) (e.g. 0, Guid.Empty, DateTime.MinValue). If obj equals the type's default, it throws ArgumentException with the parameter name. It prevents 'unset' struct values from flowing into APIs that require meaningful data.

Solutions

1. Check obj.Equals(default(T)) before the call and initialize the struct properly.
2. For Guids, ensure Guid.NewGuid() or a parsed value is assigned rather than relying on field initialization.
3. Consider making the parameter nullable (T?) so 'unset' is explicit, or change the API contract.
4. Read ex.ParamName to find which struct argument was default.

Example fix

// before
Guid id; // default Guid.Empty
Verify.IsNotDefault(id, nameof(id));
// after
Guid id = Guid.NewGuid();
if (id == Guid.Empty) throw new ArgumentException("id not initialized", nameof(id));
Verify.IsNotDefault(id, nameof(id));

if (obj.Equals(default(T)))
    throw new ArgumentException($"{nameof(obj)} must not be default(T)", nameof(obj));
Verify.IsNotDefault(obj, nameof(obj));

static bool IsSet<T>(T v) where T : struct => !default(T).Equals(v);

try {
    Verify.IsNotDefault(id, nameof(id));
} catch (ArgumentException ex) when (ex.ParamName == nameof(id)) {
    id = GenerateNewId();
}

When it happens

Trigger: Calling Verify.IsNotDefault(obj, name) where obj == default(T), e.g. an uninitialized Guid.NewGuid field, a zero id, or a struct default-initialized by deserialization.

Common situations: Passing Guid.Empty as an identifier, zero handles/ids from failed lookups, structs default-constructed instead of initialized.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• ' ' is not a valid value for ' '.
• ArgumentException(message, name)
• ArgumentNullException(name)
• ArgumentOutOfRangeException(authentication)
• ArgumentOutOfRangeException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8d966acfbd08e12c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Standard/Verify.cs:103

            {
                throw new ArgumentNullException(name, errorMessage);
            }
            if ("" == value.Trim())
            {
                throw new ArgumentException(errorMessage, name);
            }
        }

        /// <summary>Verifies that an argument is not null.</summary>
        /// <typeparam name="T">Type of the object to validate.  Must be a class.</typeparam>
        /// <param name="obj">The object to validate.</param>
        /// <param name="name">The name of the parameter that will be presented if an exception is thrown.</param>        
        [DebuggerStepThrough]
        public static void IsNotDefault<T>(T obj, string name) where T : struct
        {
            if (default(T).Equals(obj))
            {
                throw new ArgumentException("The parameter must not be the default value.", name);
            }
        }

        /// <summary>Verifies that an argument is not null.</summary>
        /// <typeparam name="T">Type of the object to validate.  Must be a class.</typeparam>
        /// <param name="obj">The object to validate.</param>
        /// <param name="name">The name of the parameter that will be presented if an exception is thrown.</param>       
        [DebuggerStepThrough]
        public static void IsNotNull<T>(T obj, string name) where T : class
        {
            if (null == obj)
            {
                throw new ArgumentNullException(name);
            }
        }

        /// <summary>Verifies that an argument is null.</summary>
        /// <typeparam name="T">Type of the object to validate.  Must be a class.</typeparam>

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