Unit Tests fail when run together, pass individually

Question

So I am having a few issues with my unit Tests, I cant just copy and past them here but I will give what I can.

The problem seems to be that if I run the tests one by one everything works as intended, but if I tell it to run the tests all together 1/5 will pass,

[TestMethod]

    public void ObjTest()
    {
        //Arrange - multiple ecus and items 
        var t = new var();
        t.itemNumbers = new List<ItemNumber>();

        obj e = new obj();
        e.property = "(12345 OR 55555) AND !65232";

        Globals.masterList.Add(e);

        ItemNumber i = new ItemNumber();
        i.num= "12345";

        ItemNumber i1 = new ItemNumber();
        i1.num= "55555";

        ItemNumber i2 = new ItemNumber();
        i2.num= "55556";
        t.itemNumbers.Add(i);
        t.itemNumbers.Add(i1); 
        t.itemNumbers.Add(i2);

        ICollection<Iinterface> tmp = new List<Iinterface>();

        //act, process the ecu and item lists
        ;
        functionCalled(t.itemNumbers, Globals.masterList, ref tmp);

        //assert, there should be only 2 added to the list
        Assert.AreEqual(1, tmp.Count, " ");
        Assert.AreEqual("(12345 OR 55555) AND !65232", functionCalled(t.itemNumbers, Globals.masterList, ref tmp), "Wrong obj returned.");

    }

All of the unit tests are basically a copy and past with the chages to e.property and possibly a change to one of the i numbers,

the tests are designed to check edge cases cased by user input.

is there something I am missing to ensure scope clears all of the variables and everything between tests. or force serial execution.

Answer 1

I propose to consider Globals.masterList.Add(e); Let's say your unit test executed in five threads. It means that Globals.masterList.Add(e); will be executed five times or masterList will be modified by five different threads. Then you have next line of code:

Assert.AreEqual("(12345 OR 55555) AND !65232", functionCalled(t.itemNumbers, Globals.masterList, ref tmp), "Wrong obj returned.");

functionCalled deals with modified list by other functions, and as outcome you have different output from it and as result failed unit test