Suffixes can be appended to the body of a lambda expression, useful in cases where you have a POCO property mapped as a multi field and want to use strongly typed access based on the property, yet append a suffix to the generated field name in order to access a particular multi field.

The .Suffix() extension method can be used for this purpose and when serializing expressions suffixed in this way, the serialized field name resolves to the last token

Expression<Func<Project, object>> expression = p => p.Name.Suffix("raw");
Expect("raw").WhenSerializing<PropertyName>(expression);

Alternatively, suffixes can be applied to a lambda expression directly using the .ApplySuffix() extension method. Again, the serialized field name resolves to the last token

Expression<Func<Project, object>> expression = p => p.Name;

expression = expression.AppendSuffix("raw");

Expect("raw").WhenSerializing<PropertyName>(expression);

Currently, the name of a field cannot contain a . in Elasticsearch due to the potential for ambiguity with a field that is mapped as a multi field.

In these cases, NEST allows the call to go to Elasticsearch, deferring the naming conventions to the server side and, in the case of a . in a field name, a 400 Bad Response is returned with a server error indicating the reason

var createIndexResponse = _client.CreateIndex("random-" + Guid.NewGuid().ToString().ToLowerInvariant(), c => c
    .Mappings(m => m
        .Map("type-with-dot", mm => mm
            .Properties(p => p
                .String(s => s
                    .Name("name-with.dot")
                )
            )
        )
    )
);

The response is not valid

createIndexResponse.ShouldNotBeValid();

DebugInformation provides an audit trail of information to help diagnose the issue

createIndexResponse.DebugInformation.Should().NotBeNullOrEmpty();

ServerError contains information about the response from Elasticsearch

createIndexResponse.ServerError.Should().NotBeNull();

createIndexResponse.ServerError.Status.Should().Be(400);

createIndexResponse.ServerError.Error.Should().NotBeNull();

createIndexResponse.ServerError.Error.RootCause.Should().NotBeNullOrEmpty();

var rootCause = createIndexResponse.ServerError.Error.RootCause[0];

We can see that the underlying reason is a . in the field name "name-with.dot"

rootCause.Reason.Should().Be("Field name [name-with.dot] cannot contain '.'");

rootCause.Type.Should().Be("mapper_parsing_exception");