Consider ModelNode.Validate<TDefinition>(Func<TDefinition, bool> callback) method

[SubPointSupport]

SPMeta2 should provide an easy way to validate the model. Something like as following:

var model = SPMeta2Model.NewSiteModel( ... );

model.Validate<FieldDefinition>( field => {
    return field.InternalName.StartsWith("myIntranet_");
});

model.Validate<FieldDefinition>( validationContext => {
    # validationContext object has tons of useful props as per the specs below
    return validationContext.Definition.InternalName.StartsWith("myIntranet_");
});

Model node should have an extension method to plug a custom validation via fun callback with the following criteria:

• Strong typing override to target particular artifact types
• Access to parent "model node" so that we would know what's the scope of the artifact is
• Access to the whole model
• Access to the collection of all artifacts of the targeted type (checks in unique ID/Name so on)

Most likely, a "validation context" is to be pass to validation callback so that more properties can be added later.

Related to community feedback on ListDefinition validation:
https://www.yammer.com/spmeta2feedback/#/Threads/show?threadId=759523765

[SubPointSupport]

First draft for the fluent validation API:

 var model = SPMeta2Model.NewSiteModel(site =>
            {
                site
                   .AddField(new FieldDefinition { InternalName = "my_field1" })
                   .AddField(new FieldDefinition { InternalName = "my_field2" })
                   .AddField(new FieldDefinition { InternalName = "my_field3" });
            });

            // all fields have "my_" prefix
            // fieldPrefixValidationResult contains a set of 'validation context' per every model node (definition) in the model
            var fieldPrefixValidationResult = model.Validate<FieldDefinition>(context =>
            {
                // context exposes three pair of props to get current, parent and child model node / definition
                // IsValid and Message is other props for the validation result

                // context.ValidationContext.ChildDefinitions
                // context.ValidationContext.ChildModelNodes

                // context.ValidationContext.CurrentDefinition
                // context.ValidationContext.CurrentModelNode

                // context.ValidationContext.ParentDefinition
                // context.ValidationContext.ParentModelNode

                // context.IsValid
                // context.Message

                context.IsValid = context.ValidationContext.CurrentDefinition.InternalName.StartsWith("my_");
            });

            Assert.IsNotNull(fieldPrefixValidationResult);
            Assert.AreEqual(true, fieldPrefixValidationResult.IsValid);

            // all fields have unique GUID
            var uniqueFieldIdValidationResult = model.Validate<SiteDefinition>(context =>
            {
                var allSiteFields = context.ValidationContext.GetChildDefinitions<FieldDefinition>();

                // all fields have non-default Ids
                // and amount of unique IDs is the same as the amount of fields 
                context.IsValid = allSiteFields.All(f => f.Id != default(Guid))
                               && allSiteFields.Select(f => f.Id).Distinct().Count() == allSiteFields.Count();

                if (!context.IsValid)
                {
                    context.Message = "Either some fields have empty ID or ID is non unique";
                }
            });

            Assert.IsNotNull(uniqueFieldIdValidationResult);

[SubPointSupport]

Implemented as per drafts early, added "Regression.Services.DefaultFluentModelValidationService" tests.

[norvegec]

Looks like in the second example (// all fields have unique GUID) enumeration of context.ValidationContext.GetChildDefinitions<FieldDefinition>() does not return any item.
But, if you remove generic constaint from model.Validate(..) method it works as expected.

[norvegec]

Here is and improved version of second example (I check for InternalName in this case):

siteFieldsModel.Validate(context =>
{
    var allSiteFields = context.ValidationContext
        .GetChildDefinitions<FieldDefinition>()
        .Select(f => f.InternalName);

    var duplicates = allSiteFields
        .GroupBy(x => x)
        .Where(x => x.Count() > 1)
        .Select(x => x.Key)
        .ToList();

    duplicates.ForEach(d => Console.WriteLine("Duplicate field:" + d));

    Assert.IsTrue(duplicates.Count == 0);
});        

[SubPointSupport]

Alright, so that is the issue if any?