ArgumentException thrown with invalid passed value, when Upload scalar is expected

[MantasK-DTS]

If a query is passed any other value (for example: a string) where an Upload scalar is expected, the IDocumentExecutor throws an unhandled ArgumentException exception. The problem is that the Upload scalar parsing accepts any value, even though it shouldn't.

Here's a relevant stacktrace when trying to pass a variable with value "foo" as the value for the Upload scalar:

System.ArgumentException: Unable to convert 'StringValue{value=foo}' to 'Upload'
   at GraphQL.Execution.ExecutionHelper.CoerceValue(ISchema schema, IGraphType type, IValue input, Variables variables) in C:\projects\graphql-dotnet\src\GraphQL\Execution\ExecutionHelper.cs:line 321
   at GraphQL.Execution.ExecutionHelper.CoerceValue(ISchema schema, IGraphType type, IValue input, Variables variables) in C:\projects\graphql-dotnet\src\GraphQL\Execution\ExecutionHelper.cs:line 306
   at GraphQL.Execution.ExecutionHelper.GetVariableValue(Document document, ISchema schema, VariableDefinition variable, Object input) in C:\projects\graphql-dotnet\src\GraphQL\Execution\ExecutionHelper.cs:line 117
   at GraphQL.Execution.ExecutionHelper.GetVariableValues(Document document, ISchema schema, VariableDefinitions variableDefinitions, Inputs inputs) in C:\projects\graphql-dotnet\src\GraphQL\Execution\ExecutionHelper.cs:line 89
   at GraphQL.DocumentExecuter.BuildExecutionContext(ISchema schema, Object root, Document document, Operation operation, Inputs inputs, IDictionary`2 userContext, CancellationToken cancellationToken, Metrics metrics, List`1 listeners, Boolean throwOnUnhandledException, Action`1 unhandledExceptionDelegate, Nullable`1 maxParallelExecutionCount, IServiceProvider requestServices) in C:\projects\graphql-dotnet\src\GraphQL\Execution\DocumentExecuter.cs:line 247
   at GraphQL.DocumentExecuter.ExecuteAsync(ExecutionOptions options) in C:\projects\graphql-dotnet\src\GraphQL\Execution\DocumentExecuter.cs:line 110

Luckily, there is an easy fix: add a cast to IFormFile in the UploadScalarType.ParseValue method:

public override object ParseValue(object value)
{
	return value as IFormFile;
}

This will make it, so that only an IFormValue is accepted. When ParseValue method returns null, graphql-dotnet will detect this as an invalid value for the type and handle it accordingly.

Another fix might be to throw exceptions on invalid values. When graphql-dotnet detects an exception in the ParseValue method, it assumes that the conversion is not possible and handles it accordingly.

[JannikLassahn]

Hi!
I feel like both approaches would make sense. Since graphql-dotnet can handle both cases we can keep it simple and use casting as you suggested.
If you would like to make a PR go right ahead, otherwise I'll fix this over the weekend.

[MantasK-DTS]

Thank you for responding! I will leave this fix up to you.