Sorry, that should be System.Runtime.Serialization.Json. System.Text.Json is the newer class that replaced it. In .Net Framework 4.6 and earlier, the only buil

Sorry, that should be System.Runtime.Serialization.Json. System.Text.Json is the newer class that replaced it. In .Net Framework 4.6 and earlier, the only built-in JSON serializer in the .Net Framework was System.Runtime.Serialization.Json.DataContractJsonSerializer. You can still see it. If you're on Windows 10, run Windows Powershell v5.1 and run: Get-Item C:\Windows\System32\notepad.exe | Select-Object -Property Name, LastWriteTime | ConvertTo-Json You'll see this output: { "Name": "notepad.exe", "LastWriteTime": "\/Date(1626957326200)\/" } Microsoft didn't fix their weird JSON serialization until quite late. They may have back ported it to the .Net Framework, but they've deleted that documentation. Powershell v6 and v7 include the newer classes that are properly behaved. This is why Json.NET used to be so popular and ubiquitous for C# and ASP applications. It generated JSON like most web applications do, not the way Microsoft's wonky class did. Indeed, I believe it may be what System.Text.Json is based on.