title: Exception in serialization of System.Data.DataTable using the new "System.Text.Json" class (Asp.net core 3.0 preview 8)
body: <p>I am writing a rest api in asp.net core 3.0 preview 8 and I was trying to serialize a System.Data.DataTable using the new "System.Text.Json" class, but in the Serialize method I receive the exception:</p>
  
  <blockquote>
    <p>The collection type 'System.Data.DataRelationCollection' on 'System.Data.DataTable.ChildRelations' is not supported. </p>
  </blockquote>
  
  <p>The same serialization works well using the newtonsoft json serializer.</p>
  
  <p>Sample code to reproduce the problem:</p>
  
  <pre><code>var dt = new System.Data.DataTable("test");
  dt.Columns.Add("Column1");
  var ser=System.Text.Json.JsonSerializer.Serialize(dt);
  </code></pre>
  
  <p>Detailed exception:</p>
  
  <blockquote>
    <p>System.NotSupportedException
      HResult=0x80131515
      Message=The collection type 'System.Data.DataRelationCollection' on 'System.Data.DataTable.ChildRelations' is not supported.
      Source=System.Text.Json
      StackTrace:
       at System.Text.Json.JsonClassInfo.GetElementType(Type propertyType, Type parentType, MemberInfo memberInfo, JsonSerializerOptions options)
       at System.Text.Json.JsonClassInfo.CreateProperty(Type declaredPropertyType, Type runtimePropertyType, Type implementedPropertyType, PropertyInfo propertyInfo, Type parentClassType, JsonSerializerOptions options)
       at System.Text.Json.JsonClassInfo.AddProperty(Type propertyType, PropertyInfo propertyInfo, Type classType, JsonSerializerOptions options)
       at System.Text.Json.JsonClassInfo..ctor(Type type, JsonSerializerOptions options)
       at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type classType)
       at System.Text.Json.WriteStackFrame.Initialize(Type type, JsonSerializerOptions options)
       at System.Text.Json.JsonSerializer.WriteCore(Utf8JsonWriter writer, PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)
       at System.Text.Json.JsonSerializer.WriteCore(PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)
       at System.Text.Json.JsonSerializer.WriteCoreString(Object value, Type type, JsonSerializerOptions options)
       at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
       at ErrorJsonMIcrosoftDataTable.Controllers.WeatherForecastController.Get() in G:\testnet\ErrorJsonMIcrosoftDataTable\ErrorJsonMIcrosoftDataTable\Controllers\WeatherForecastController.cs:line 31
       at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)
       at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
       at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.&lt;g__Logged|12_1>d.MoveNext()</p>
  </blockquote>
  
  <p>May you kindly help?</p>
  
  <p>Thank you.</p>