为什么LINQ无法转换string.IsNullOrWhiteSpace（）？

Question

Can someone explain why this compiles but creates an exception at runtime because it is not able to translate to sql/entities correctly? Trying to gain understanding of what is happening behind the scenes.

 var data = (from e in db.Employees
            join egp in db.EmployeeGrievanceMappings on e.EmployeeID equals egp.EmployeeID
            join et in db.EmployeeTypes on egp.EmployeeTypeID equals et.EmployeeTypeID
                    where et.EmployeeTypeName == employeeeType
            select new GrievantStewardBO()
            {
                Id = e.EmployeeADID,
                EmployeeID = e.EmployeeID,
                EmployeeTypeID = egp.EmployeeTypeID,
                GrievanceID = egp.GrievanceID,
                EmployeeGrievanceID = egp.EmployeeGrievanceID,
                text =  string.IsNullOrWhiteSpace( e.EmployeeLastName) ? e.EmployeeFirstName : e.EmployeeLastName + "," + e.EmployeeFirstName,
                FirstName = e.EmployeeFirstName,
                LastName = e.EmployeeLastName

            }).
        ToList();

I know I can write this and have it work correctly, I just wanted to gain some understanding for similar run-ins with LINQ.

text =   e.EmployeeLastName == null || e.EmployeeLastName.Trim() == "" ? e.EmployeeFirstName : e.EmployeeLastName + "," + e.EmployeeFirstName

Related question: Why does Trim() translate correctly in LINQ?

Answer 1

You are using "Linq to Entites", likely either Entity Framework or Linq2Sql. When using Linq to Entites your linq statement is turned in to a expression, parsed, then is converted in to a SQL query. The problem you are having is as simple as the person who wrote the "parse" step did not support string.IsNullOrWhiteSpace in the parser so it does not know how to turn that function in to a SQL string.

You must either use the long form like you showed or update to a newer version of your library that supports it. It appears that the version on GitHub of Entity Framework 6 does IsNullOrEmpty , perhaps you could create a new method of IsNullOrWhitespace and do a request to get it merged in.

Answer 2

The LINQ you have there is an expression which gets translated into SQL. Somebody has to evaluate the expression and create the resulting SQL. This process does not know what to do with IsNullOrWhiteSpace() , hence it will fail.

Answer 3

Why? Nobody wrote code to handle this specific method in the LINQ provider you are using. Method calls are not translated to SQL (or whatever the provider translates to) by some procedure. Method calls are handled on a case by case basis and the common methods are hardcoded into the provider. If nobody wrote code to handle this method the method does not work.

On another note why didn't they handle this specific method? Probably they were either reusing code from LINQ to SQL for their generation code or they wanted to work with .NET framework 3.5 and this method was added in 4.0