interface IValid
{
    bool IsValid();
}

public abstract class BaseClass : IValid
{
    public bool Validity { get; protected set; } = true;

    public bool IsValid()
    {
        return this.Validity;
    }
}

public class ChildOfTrue : BaseClass
{
    public ChildOfTrue() => this.Validity = true;
}

public class ChildOfFalse : BaseClass
{
    public ChildOfFalse() => this.Validity = false;
}

public class TestCase
{
    public TestCase()
    {
        List<BaseClass> testSubjects = new List<BaseClass>
        {
            new ChildOfTrue(),
            new ChildOfFalse(),
            null
        };
        foreach (var testSubject in testSubjects)
        {
            if (testSubject != null && testSubject.IsValid())
            {
                /* Perform tests. */
            }
        }
    }
}

public static class BaseClassExtensionHelper
{
    public static bool IsValid(this BaseClass o)
    {
        if (ReferenceEquals(o, null)) return false;
        return o.Validity;
    }

    public static bool IsNotNullAndValid(this BaseClass o)
    {
        if (ReferenceEquals(o, null)) return false;
        return o.IsValid();
    }
}

if (BaseClassExtensionHelper.IsValid(testSubject)) { /* Perform tests. */ }