Updated System.Text.Json and System.Diagnostics.DiagnosticSource package references to version 10.0.0. Simplified the condition to check for TargetFramework not equal to net8.0.

When using Elastic.Clients.Elasticsearch 8.19.12 (NuGet) with System.Text.Json 10.0.0, a runtime MissingMethodException occurs:

System.MissingMethodException: Method not found:
'Boolean Elastic.Transport.Extensions.TransportSerializerExtensions.TryGetJsonSerializerOptions(...)'

Root Cause

The NuGet package Elastic.Transport 0.10.1 was compiled against System.Text.Json 8.0.5. When the application loads System.Text.Json 10.0.0 at runtime, there is a binary incompatibility due to internal breaking changes in
System.Text.Json (specifically in internal types like PooledByteBufferWriter).

The issue manifests during JSON serialization when Elastic.Transport calls JsonSerializer.SerializeAsync(), which internally uses types whose implementation changed between versions 8.x and 10.0.

Solution

Recompile Elastic.Transport and Elastic.Clients.Elasticsearch against System.Text.Json 10.0.0. This ensures the compiled IL references the correct internal type implementations.

Changes Made

elastic-transport-net (Elastic.Transport.csproj):

Key Takeaway

System.Text.Json 10.0.0 has internal breaking changes that require dependent libraries to be recompiled - simply updating the runtime DLL without recompiling causes MissingMethodException errors.