You might have had a situation where your code once worked fine in ASP.NET application but now throws the below error.
System.MissingMethodException: Method not found
I have noticed this more recently in the Outsystems Web Application.
One of the main reason for this error is that there is an old version of the DLL that might be referenced or still been used somewhere with your code.
How to Fix System.MissingMethodException: Method not found in C# ?
To fix the Method not found error in C#, ensure that the latest assemblies are deployed and no duplicate or older assemblies are existing in any of the sub folders with-in your application.
When Deploying, also purge the entire folder before you deploy your .NET application.