Fix ConvertTypeCheckPatternToNullCheck

[xtqqczze]

Use null check pattern instead of a type check succeeding on any not-null value.

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[huoyaoyuan]

This is not readable. In the initial version of pattern matching before is not null, is { } is rejected by many projects.

[xtqqczze]

We cannot use is not null with declaration pattern, i.e. if (objAttr[i] is not null attr) is invalid C#.

[xtqqczze]

Suggested change

	if (objAttr[i] is { } attr)
	if (objAttr[i] is not null and var attr)

[jkotas]

Yes, it is unfortunate that there are number of more or less cryptic ways one can express a null check in modern C#. The choice between them is a matter of personal preference. My personal preference is the old fashioned:

object? attr = objAttr[i];
if (attr != null)

I believe that it is the best balance of readable and succinct. (Except for the few types that overload operator==. It is fine to use more verbose is not null in those cases for performance reasons.)

[xtqqczze]

Suggested change

	if (Environment.GetEnvironmentVariableCore_NoArrayPool(variable) is { } envVar &&
	envVar.Length is > 0 and <= 32) // arbitrary limit that allows for some spaces around the maximum length of a non-negative Int32 (10 digits)
	string? envVar = Environment.GetEnvironmentVariableCore_NoArrayPool(variable);
	if (envVar is { Length: > 0 and <= 32 }) // arbitrary limit that allows for some spaces around the maximum length of a non-negative Int32 (10 digits)

@jkotas what is your opinion on this pattern?

[jkotas]

I think it is about as good as the pattern in main. Some people prefer multiline statements, some people prefer single line statements. (Personally, I tend to prefer single line statements when everything else is equal.)