Do not access property multiple times in same method

[RalfKoban]

It may happen that programmers write code as following:

public object SomeProperty { get; set; }

public void DoSomething()
{
    if (SomeProperty == null)
    {
    }

    if (SomeProperty is IAbc)
    {
    }

    if (SomeProperty is IXyz)
    {
    }
}

This leads to race-conditions in case SomeProperty gets changed while being inside the method.

Hence, they should assign the result of SomeProperty to a local variable and use that instead.

public object SomeProperty { get; set; }

public void DoSomething()
{
    var value = SomeProperty;

    if (value == null)
    {
    }

    if (value is IAbc)
    {
    }

    if (value is IXyz)
    {
    }
}

[RalfKoban]

Same applies for code like:

public void DoSomething()
{
    if (Abc.Def == null)
    {
        return;
    }

    foreach (var x in Abc.Def)
    { ... }
}

where Abc.Def represents an access to some properties.

[RalfKoban]

We should also check for coalescending access, such as in: if (Abc?.Def?.Ghi != null)