Caching expressions with "string.Equals()" does not work

[maximgorbatyuk]

If we cache an expression that contains string.Equals(string), a result will be the same even any kind of string value would be passed as arguments. But a way string == string works.

[VahidN]

I added a test for it and I can't reproduce it. IsCacheHit is always false here, which means it doesn't read the value from the cache, when it's using different random values.

[maximgorbatyuk]

Sorry for late response, @VahidN . I have tried some ways to reproduce an it seems like the bug will appear when I use "Invariant" comparing. Look to the screenshots below: When I use `string.Equals("ololo", InvariantCultureIgnoreCase) the bug appears:

When I use string.ToUpperInvariant() == string.ToUpperInvariant() the bug appears:

When I use just string.Lower() == string.Lower() the bug does not appear:

Here is a piece of code to make yor way to reproduce the bug more easy: https://gist.github.com/maximgorbatyuk/4a48e227e7f47d3097860b58678059f0

[VahidN]

• You should start reading about the Collations. When you are working with for instance the SQL Server, you don't need to use ToUpper or ToLower in your queries. Why? Because the SQL Server will use its current Collation to handle it. more info
• When you are using ToUpperInvariant in your Linq to Entities queries, EF will switch to the client side evaluation to handle it. in this case, you think it will add your parameters to the final SQL, but it won't. It will create a general SQL without any parameters and then it will filter the result on the client side. that's why you will see that behavior. Because without the parameters, hash of the SQL string won't change and this hash means the cache key. More info.
• If you insist to use the client side evaluation, you can use the saltKey parameter, to change the final hash and make it unique.
• You can add ConfigureWarnings(warnings => warnings.Throw(RelationalEventId.QueryClientEvaluationWarning) to your EF Core settings to see which queries are being evaluated on the client, not on the server.

[maximgorbatyuk]

Oh, thank you for links to the materials. We use PostgresQL instead of MSSQL server. So, to make it clear, we should use just .Where(x => x.Title == title) to search with insensitive case in LinqToSql even title has some kind of "pOsT1"value, isn't it?

[VahidN]

• PostgreSQL is different than SQL Server and you will need tolower for it.
• In this case, use methods that are not being evaluated on the client side and can be translated to SQL (for performance reasons and for better caching).
• If you insist to use the client side evaluation, you can use the saltKey parameter, to change the final hash and make it unique.
• But remember, LINQ to Entities queries are different than LINQ to Object queries, because of the Collation concept.

[maximgorbatyuk]

Thank you for explanation of background!

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