C # constant expression constant

Question

Can someone explain why if I ran this expression:

const string testValue = "ABC"; 
return NameDbContext.MasterNames
    .Where(m => m.Names.Any(n => n.LastName == testValue))
    .ToList();

I get the expected result, however, if I run the same with testValuea variable, it fails:

string testValue = "ABC"; 
return NameDbContext.MasterNames
    .Where(m => m.Names.Any(n => n.LastName == testValue))
    .ToList();

This only happens with string. Similar code with intworked fine with testValueeither a variable or a constant.

I suspect this is due to the object nature of the string. If so, then how can I call this expression with a variable (I do not know the value testValueat compile time).

Thank.

EDIT:

( > 6 ). . , where ( ).

EDIT2:

:

:

SELECT *
  FROM (SELECT   "Filter2"."MALPHA_KEY" AS "MALPHA_KEY"
      FROM (SELECT "Extent1"."MALPHA_KEY" AS "MALPHA_KEY",
          ROW_NUMBER () OVER (ORDER BY "Extent1"."MALPHA_KEY" ASC)
                                                              AS "row_number"
                    FROM "RMS"."ALPHA_MASTER_NAME" "Extent1"
                   WHERE (EXISTS (
                             SELECT 1 AS "C1"
                               FROM "RMS"."ALPHA" "Extent2"
                              WHERE (    ("Extent1"."MALPHA_KEY" =
                                                        "Extent2"."MALPHA_KEY"
                                         )
                                     AND ('ABC' = "Extent2"."LAST_NAME")
                                    ))
                         )) "Filter2"
           WHERE ("Filter2"."row_number" > 0)
        ORDER BY "Filter2"."MALPHA_KEY" ASC)
 WHERE (ROWNUM <= (50))

:

SELECT *
  FROM (SELECT   "Project2"."MALPHA_KEY" AS "MALPHA_KEY"
            FROM (SELECT "Project2"."MALPHA_KEY" AS "MALPHA_KEY",
                         ROW_NUMBER () OVER (ORDER BY "Project2"."MALPHA_KEY" ASC)
                                                              AS "row_number"
                    FROM (SELECT "Extent1"."MALPHA_KEY" AS "MALPHA_KEY"
                            FROM "RMS"."ALPHA_MASTER_NAME" "Extent1"
                           WHERE (EXISTS (
                                     SELECT 1 AS "C1"
                                       FROM "RMS"."ALPHA" "Extent2"
                                      WHERE (    ("Extent1"."MALPHA_KEY" =
                                                        "Extent2"."MALPHA_KEY"
                                                 )
                                             AND (   ("Extent2"."LAST_NAME" =
                                                                   :p__linq__0
                                                     )
                                                  OR (    ("Extent2"."LAST_NAME" IS NULL
                                                          )
                                                      AND (:p__linq__0 IS NULL
                                                          )
                                                     )
                                                 )
                                            ))
                                 )) "Project2") "Project2"
           WHERE ("Project2"."row_number" > 0)
        ORDER BY "Project2"."MALPHA_KEY" ASC)
 WHERE (ROWNUM <= (50))

where ( ), NULL

    AND (   ("Extent2"."LAST_NAME" = :p__linq__0
        )
   OR (    ("Extent2"."LAST_NAME" IS NULL )
   AND (:p__linq__0 IS NULL )  )  )

NULL ...

Answer 1

# 1

, null , , :

NameDbContext.Configuration.UseDatabaseNullSemantics = true;

WHERE:

WHERE "Extent2"."LAST_NAME" = :p__linq__0

, , , NameDbContext.

@IanMercer node, WHERE. , , , Oracle , , .

# 2

( SQL Server, , , ), , LAST_NAME :p__linq__0. -unicode, , EF, unicode (varchar vs nvarchar - unicode EF), .

Answer 2

- ExpressionVisitor, .

, , ( ):

 Expression<Func<int, int, bool>> expr = (a, b) => a < b;
 var applied = expr.Apply(input.FirstMonth);

( ) Apply ( ):

/// <summary>
/// Partially apply a value to an expression
/// </summary>
public static Expression<Func<U, bool>> Apply<T, U>(this Expression<Func<T, U, bool>> input,
    T value)
{
   var swap = new ExpressionSubstitute(input.Parameters[0],
       Expression.Constant(value));
   var lambda = Expression.Lambda<Func<U, bool>>(
       swap.Visit(input.Body), 
       input.Parameters[1]);
   return lambda;
}


class ExpressionSubstitute : System.Linq.Expressions.ExpressionVisitor
{
    private readonly Expression from, to;
    public ExpressionSubstitute(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }

    public override Expression Visit(Expression node)
    {
        if (node == from) return to;
        return base.Visit(node);
    }
}

Answer 3

linq, . :

Expression<Func<Name, bool>> exp1 = name => name.LastName == testValue;
Expression<Func<MasterName, bool>> exp2 = masterName => masterName.Names.Any(exp1);
var result = NameDbContext.MasterNames.Where(exp2).ToList();

:

"" , . # .

, - ConstantExpression.

The easiest way to create expression trees is usually to create something similar in a lambda expression, and then look at the generated code in Reflector by turning the optimization level down so that Reflector does not convert it to lambda expressions.

If I define a local varaible string testValue = "ABC";, the debug view is displayed:

.Lambda #Lambda1<System.Func`2[ConsoleApp.Program+Name,System.Boolean]>(ConsoleApp.Program+Name $name)
{
    $name.LastName == .Constant<ConsoleApp.Program+<>c__DisplayClass0_0>(ConsoleApp.Program+<>c__DisplayClass0_0).testValue
}

Now, if I define a constant const string testValue = "ABC";, the debug view is displayed:

.Lambda #Lambda1<System.Func`2[ConsoleApp.Program+Name,System.Boolean]>(ConsoleApp.Program+Name $name)
{
    $name.LastName == "ABC"
}