Nullable properties `NullReferenceException` not thrown. about linqkit HOT 3 OPEN

If you are building predicate for EF you are doing this in wrong way. PredicateBuilder will not help here.
Create question on StackOverflow with detailed description what you ate trying to achieve.

from linqkit.

@sdanyliv I am indeed doing that, I followed the demo you had in the Readme calling AsExpandable() and using predicate.Compile() everything seems to be working fine I don't see what's wrong with that? It's just this single problem that was stopping it from working on some cases because no error was thrown.
Here's the link to the demo app I made if you want to test it.

from linqkit.

@elf-is You are referring to DataTableService.SearchAColumn? I believe, that you misunderstand the behaviour of EF Core and your Database here, and there isn't a lot LINQKit can do about this. It may also be, that you would need to push for a C# Language specification change.

So first, what does EF Core do with Expressions?

1. EF Core tries to translate C#-function-calls into SQL
2. It seems you are using Pomelo.EntityFrameworkCore.MySql as database provider, that documents that stringValue.Contains(value) is translated into expr LIKE pat [ESCAPE 'escape_char'] or LOCATE(substr,str). See https://github.com/PomeloFoundation/Pomelo.EntityFrameworkCore.MySql/wiki/Translations#string-functions
3. MySQL states for both, that if any argument is NULL, the result is NULL. There is no exception/error here and NULL for most aspects behaves like false in boolean-context

Second, if you intend to use the predicate outside EF Core you basically hit the limits of the C# compiler/C# Language specification. For all I know you can only use the expressions present in .NET 3.5 directly in source code and have it translated for you into an Expression tree, see also https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/expression-trees/#limitations . What you can do right now, is to write your own helper like

    public class CoalescingTests
    {

        [Theory]
        [InlineData(null, false)]
        [InlineData("x", true)]
        [InlineData("y", false)]
        public void NullCoalesce(string value, bool expectation)
        {
            Expression<Func<string, bool>> expression = s => s.IfNotNull(str => str.Contains("x"), () => false);
            Assert.Equal(expectation, expression.Invoke(value));

            var expanded = expression.Expand();
            Assert.DoesNotContain("IfNotNull", expanded.ToString());
            Assert.Equal(expectation, expanded.Invoke(value));
        }
    }

    public static class CoalescingExtensions
    {
        [Expandable(nameof(IfNotNullWithDefault))]
        public static TResult IfNotNull<TIn, TResult>(this TIn value, Expression<Func<TIn, TResult>> then, Expression<Func<TResult>> @else)
        {
            return value != null ? then.Invoke(value) : @else.Invoke();
        }

        private static Expression<Func<TIn, Expression<Func<TIn, TResult>>, Expression<Func<TResult>>, TResult>> IfNotNullWithDefault<TIn, TResult>()
        {
            return (value, then, @else) => value != null ? then.Invoke(value) : @else.Invoke();
        }
    }

Calling Expand() results in s => IIF((s != null), s.Contains("x"), False). This differs from ?. in that it does not define a variable for the checked s. If s actually is a computed property, you may want to modify the expansion, to first assign the value to a local variable, as otherwise expanding will spread the property access to all places. Given that you build your predicate it may however be better memory-wise to create the temporary variable once and use it in all predicates.

This would be an expansion with local variable (not supported for .NET 3.5):

        private static Expression<Func<TIn, Expression<Func<TIn, TResult>>, Expression<Func<TResult>>, TResult>> IfNotNullWithDefault<TIn, TResult>()
        {
            var valueParameter = Expression.Parameter(typeof(TIn), "value");
            var value = Expression.Parameter(typeof(TIn), "v");
            var then = Expression.Parameter(typeof(Expression<Func<TIn, TResult>>), "then");
            var @else = Expression.Parameter(typeof(Expression<Func<TResult>>), "else");
            return Expression.Lambda<Func<TIn, Expression<Func<TIn, TResult>>, Expression<Func<TResult>>, TResult>>(
                Expression.Block(
                    new[] {value},
                    Expression.Assign(value, valueParameter),
                    Expression.Condition(
                        Expression.NotEqual(value, Expression.Constant(null, typeof(TIn))),
                        Expression.Invoke(then, value),
                        Expression.Invoke(@else))),
                valueParameter, then, @else
            );
        }

Update: tried to provide shorter code-samples. Defining & assigning variables sadly isn't supported in expression-trees, so it takes up more lines

from linqkit.