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InvalidOperationException: An exception was thrown while attempting to evaluate the LINQ query parameter expression


What is a LINQ Query?

Linq which stands for Language Integrated Query is a Framework Library that allows Software Developers to write C-Sharp (C#) code to query data. Linq code is then interpreted into SQL Language that most Databases understand.                                                                                                                                           

Question: How do you solve for the error in AspNet 6 Entity Framework (EF Core 6) that says "InvalidOperationException: An exception was thrown while attempting to evaluate the LINQ query parameter expression"?

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Answer:
It is a good practice to pass in the parameter in the Linq Query so that the Entity Framework does not have to compile the Query Tree every time the request is passed. However, in passing in the parameter in the Linq Query you have to make sure that you handle Null Exceptions or InvalidOperationExceptions by checking if the variable (parameter passing in the Linq Query) has the correct data type. 

[Rule of Thumb]: The less you minimize Exception Handling the faster the code runs. First, strive for ways for the code to just work, if not possible then handle Exceptions with a Try-Catch block or by checking for null. Another efficient way to check for Nulls is by utilizing Nullables in C# 10.

In the case above, if you are passing in the ID of the Model into the Linq Query for Entity Framework to retrieve the record from the database, and the ID comes in as a String, then you need to explicitly check or convert the String to Int32 or vise versa.

You need to Convert the Lambda Expression ID (e.g. _context.MyPOCO.Where(a => a.ID.ToString() == ID)) to a String to match the ID coming in as a String.

[Note]: Doing a lot of Conversion on the Linq Query might slow down the compilation of the Query Tree by EF Core 6, instead, I would advise cleaning parameter data and then using the defined variables with the parameter values in the Linq Query.



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