NullReferenceException when applying custom aggregation over dynamic (open type) properties

[NiazMohy-ud-din]

Assemblies affected
Microsoft.AspNetCore.OData 9.3.0

Describe the bug
Custom aggregate function e.g. $apply=aggregate(... with custom.stdev) over dynamic properties (open type dictionary fields) throws NullReferenceException during parsing. The options ODataQueryOptions.Apply.ApplyClause contains the exception as soon as the action is called.

I can verify that the declared properties work fine, and I've successfully parsed custom.stdev using following query, where filesize is part of the entity:
$apply=aggregate(filesize with custom.stdev as filesizedev)

However, when the aggregated field is a dynamic (open type) property, the parser fails.
$apply=aggregate(dynprop with custom.stdev as dynpropdev)
Interesting is, if I add dynprop to EdmModel like following then the parsing of ApplyClause works:

var edmEntityType = (EdmEntityType) model.FindDeclaredType(typeof(MyEntity).FullName);
edmEntityType.AddStructuralProperty("dynprop", EdmPrimitiveTypeKind.Double);

Unfortunately, I can't do that, as the properties are dynamic and depend on the data.

Is there a supported way to dynamically provide type information for open-type properties at runtime, before the ODataQueryOptions is prepared for controller action?

If not, is there a recommended approach or extension point to safely support this scenario?

[NiazMohy-ud-din]

Additionally, I have also tried following but no success:

var stdevMethodAnnotation = new CustomAggregateMethodAnnotation();
var stdevMethod = typeof(CustomAggregates).GetMethod("StdDev", BindingFlags.Static | BindingFlags.Public);
stdevMethodAnnotation.AddMethod("custom.stdev", new Dictionary<Type, MethodInfo>
{
{ typeof(object), stdevMethod }
});
model.SetAnnotationValue(model, stdevMethodAnnotation);

[NiazMohy-ud-din]

An easy way to reproduce using unit test:

var model = EntityDataModel.GetModel();
var entityType = model.FindDeclaredType(typeof(OEntity).FullName) as IEdmEntityType;
var entitySet = model.EntityContainer.FindEntitySet("Files");

string apply = "aggregate(dynprop with custom.stdev as dynpropdev)";
var queryOptionParser = new ODataQueryOptionParser(model, entityType, entitySet, new Dictionary<string, string> { { "$apply", apply } });
var applyClause = queryOptionParser.ParseApply(); // Here Exception!

[gathogojr]

Thank you, @NiazMohy-ud-din, for reporting this issue.

You're absolutely right—this is a bug, and it can be traced to https://github.com/OData/odata.net/blob/5d1549414fc9b348ebd6fa2f5fd2f1fb5bcf1601/src/Microsoft.OData.Core/UriParser/Aggregation/ApplyBinder.cs#L222 in the OData.NET repo.

The root cause lies in how dynamic properties are handled: expressionType is null for dynamic properties. As a result, when GetCustomMethodReturnType is invoked - even if the custom aggregation method is properly registered and added to the EDM model as an instance annotation - the return type cannot be resolved. This is because the current logic relies on expressionType to determine the expected (or "source") type and match it against the function signature using:

if (CanPromoteParameterTo(expressionType, parameterType))

This limitation is acknowledged in the code with a TODO comment, indicating a known feature gap:

// Only the EdmModel knows which type the custom aggregation methods returns.
// Since we do not have a reference for it, right now we are assuming that all custom aggregation methods returns Doubles
// TODO: find a appropriate way of getting the return type.

To reproduce the issue, even when registering a custom aggregation method like this:

CustomUriFunctions.AddCustomUriFunction("custom.stdev", new FunctionSignatureWithReturnType(
    EdmCoreModel.Instance.GetDouble(false),
    EdmCoreModel.Instance.GetDecimal(false)));

…the return type resolution still fails due to the missing expressionType.

To address this, we need to implement a strategy that allows the return type to be resolved independently of expressionType, possibly by leveraging metadata from the EDM model or enhancing the binding logic to support dynamic scenarios.

[NiazMohy-ud-din]

Hello @OData Team,

Could you please look into this issue or suggest a possible workaround?
We need to implement some custom functions for our customers (e.g. running totals, last), which are currently not achievable using standard OData aggregation.

Your assistance would be greatly appreciated.
Thank you in advance!