Unclear exception when providing Default = null for an option

[ptylenda]

In 2.x version of CommandLine it is not possible to specify null as default value of an option:

[Option('u', "user", Default = null, HelpText = "User to run the schedule as")]
public string User{get; set}

You will get such an exception:

Unhandled Exception: System.Reflection.CustomAttributeFormatException: 'Default'
 property specified was not found. ---> System.Reflection.TargetInvocationExcept
ion: Exception has been thrown by the target of an invocation. ---> System.Argum
entNullException: Value cannot be null.
Parameter name: value
 at CommandLine.BaseAttribute.set_Default(Object value)
 --- End of inner exception stack trace ---
 at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments,
 Signature sig, Boolean constructor)
 at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Objec
t[] parameters, Object[] arguments)
 at System.Reflection.RuntimeMethodInfo.UnsafeInvoke(Object obj, BindingFlags
invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
 at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decora
tedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFi
lterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecorate
dTargetSecurityTransparent)
 --- End of inner exception stack trace ---
 at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decora
tedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFi
lterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecorate
dTargetSecurityTransparent)
 at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimePropertyInfo
property, RuntimeType caType)
 at System.Reflection.RuntimePropertyInfo.GetCustomAttributes(Boolean inherit)

 at CommandLine.Core.ReflectionExtensions.<>c__0`1.<GetSpecifications>b__0_1(P
ropertyInfo pi)
 at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
 at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
 at System.Linq.Lookup`2.Create[TSource](IEnumerable`1 source, Func`2 keySelec
tor, Func`2 elementSelector, IEqualityComparer`1 comparer)
 at System.Linq.GroupedEnumerable`3.GetEnumerator()
 at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
 at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate
)
 at CommandLine.Core.SpecificationExtensions.ThrowingValidate(IEnumerable`1 sp
ecifications, IEnumerable`1 guardsLookup)
 at CommandLine.Core.InstanceBuilder.Build[T](Maybe`1 factory, Func`3 tokenize
r, IEnumerable`1 arguments, StringComparer nameComparer, CultureInfo parsingCult
ure, IEnumerable`1 nonFatalErrors)
 at CommandLine.Parser.ParseArguments[T](IEnumerable`1 args)

The behaviour is perfectly fine, but it is not documented and the exception does not clearly state which option property caused the exception. Second thing is that using Default = null was possible in 1.x, so while migrating 1.x -> 2.x you will run into this unexpected exception and it is not immediately clear where to look for the root cause (especially if you have a large number of option properties)

[anthonylangsworth]

@ptylenda, from what I can tell, there is no reason why null default values should not be allowed. I suspect is is overzealous precondition checking and have submitted a pull request to remove the check.

[nemec]

The pull request has been merged