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The following compiles on .NET, but not Mono:
static void Main(string args)
static void SomeFunc([Required] int someParam)
The error when compiling on Mono is "/home/greg/dev/RequiredAttributeTest/RequiredAttributeTest/Main.cs(31,31): Error CS0592: The attribute `System.ComponentModel.DataAnnotations.RequiredAttribute' is not valid on this declaration type. It is valid on `property, indexer, field' declarations only (CS0592) (RequiredAttributeTest)"
It appears that [Required] became applicable to parameters in .NET 4.0. I ran into this while trying to test an ASP.NET MVC application on Mono. I assume all that's needed is to change the AttributeUsage of RequiredAttribute.
Fixed in master