I want to create predicate expression so that it translates into WHERE clause with SQL parameter, eg:
WHERE e.Id = @id_1;
I have this code
int id = 1;
Expression<Func<Entity, int>> keySelector = e => e.Id;
BinaryExpression equals = Expression.Equal(
keySelector.Body,
Expression.Constant(id, keySelector.Body.Type)); //replace this with a variable
var predicate = Expression.Lambda<Func<Entity, bool>>(equals, keySelector.Parameters[0]);
but it translates into:
WHERE e.Id = 1;
which generates new query execution plan for each id.
Does EF core provide some internal mechanism, that I could (should) use, eg special subclass of Expression, or ExpressionVisitor?
Does EF core provide some internal mechanism, that I could (should) use, eg special subclass of Expression, or ExpressionVisitor?
Nothing special. All you need is to emulate C# closure. This could be done in several ways - using anonymous type, concrete type, Tuple
or even real C# compiler generated closure like
int id = 1;
Expression<Func<int>> valueSelector = () => id;
var value = valueSelector.Body; // use this in place of Expression.Constant
Example of anonymous type approach
int id = 1;
var variable = new { id };
var value = Expression.Property(Expression.Constant(variable), nameof(id));
Similar with new Tuple<int>(id)
and Item1
.
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