How to check for null on Nullable types using Linq expression in Realm?

问题

I want to get all elements where the Modified property isn't set but can't seem to get it to work with Realm.

Sample Code:

public class FooModel : RealmObject
{
  public DateTimeOffset? Modified { get; set; }
}

...

public List<FooModel> GetAllUnmodified()
{
  var realm = Realm.GetInstance();

  //doesn't work
  var result1 = realm.All<FooModel>().Where(model => model.Modified == null).ToList();

  //doesn't work
  var result2 = realm.All<FooModel>().Where(model => !model.Modified.HasValue).ToList();

  //doesn't work
  DateTimeOffset? testValue = null;
  var result3 = realm.All<FooModel>().Where(model => model.Modified == testValue).ToList();

  //doesn't work
  var result4 = realm.All<FooModel>().Where(model => model.Modified == default(DateTimeOffset?)).ToList();

  return result1;
}

Always getting System.NotSupportedException: The rhs of the binary operator 'Equal' should be a constant or closure variable expression. or System.NotSupportedException: The member 'HasValue' is not supported

Did I miss anything? Is there a good way to see what actually is supported by Realm's Linq?

Using Realm Xamarin v0.77.1 on Android

EDIT:

I did try creating a linq expression tree as suggested by a commenter. This resulted in a System.MissingMethodException: Method 'RealmResults'1.get_Provider' not found. exception.

回答1:

This feature is missing and high priority: #517. We are very aware that we have a number of LINQ shortcomings, we are looking into that whole area, including writing a summary about what features are supported, in the near future.

回答2:

Note for anyone seeing this later - this feature was added in version 0.77.0 and is live in the current 0.78.1.

We now support comparison to null.

public class Person : RealmObject
{
    public bool? IsAmbivalent { get; set; }
...
_realm.All<Person>().Where(p => p.IsAmbivalent == null);

Or, for string properties, also checking:

_realm.All<Person>().Where(p => string.IsNullOrEmpty(p.OptionalAddress));

See unit tests for more examples

来源：https://stackoverflow.com/questions/38763893/how-to-check-for-null-on-nullable-types-using-linq-expression-in-realm

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linq

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realm

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