Example<\/h1>\n\n\n\n
Let’s consider the following code snippet:<\/p>\n\n\n\n
public class Closure\n{\n public static Func<int> GetFuncWithClosure(int arg)\n {\n int internalVariable = arg;\n\n var func = new Func<int>(() =>\n {\n internalVariable -= 5;\n\n return 10 \/ internalVariable;\n });\n\n return func;\n }\n\n public static Func<int, int> GetFuncWithoutClosure()\n {\n var func = new Func<int, int>((arg) =>\n {\n arg -= 5;\n\n return 10 \/ arg;\n });\n\n return func;\n }\n}<\/code><\/pre>\n\n\n\nAs you can see there’s one simple class named Closure<\/code> and two static methods named GetFuncWithClosure<\/code> and GetFuncWithoutClosure<\/code>. First method returns the Func<int> – in other words, a function that accepts no argument and returns an integer. Second method returns the Func<int, int> – a function that accepts one argument and returns an integer. There’s one caveat in terms of GetFuncWithClosure<\/code> though, returned function uses the variable named internalVariable<\/code>, hence closure.<\/p>\n\n\n\nTests<\/h1>\n\n\n\n
Having our program ready to go, let’s focus on a few unit tests.<\/p>\n\n\n\n
[Fact]\npublic void GetFuncWithClosure_InvokeOnce_GetExpectedResult()\n{\n \/\/arrange\n var func = Closure.GetFuncWithClosure(10);\n\n \/\/act\n var result = func();\n\n \/\/assert\n Assert.Equal(2, result);\n}\n\n[Fact]\npublic void GetFuncWithClosure_InvokeMoreThanOnce_GetUnexpectedResult()\n{\n \/\/arrange\n var func = Closure.GetFuncWithClosure(10);\n\n \/\/act & assert\n var result = func();\n Assert.Equal(2, result);\n\n Assert.Throws<DivideByZeroException>(() => func());\n}\n\n[Fact]\npublic void GetFuncWithoutClosure_InvokeOnce_GetExpectedResult()\n{\n \/\/arrange\n var func = Closure.GetFuncWithoutClosure();\n\n \/\/act\n var result = func(10);\n\n \/\/assert\n Assert.Equal(2, result);\n}\n\n[Fact]\npublic void GetFuncWithoutClosure_InvokeMoreThanOnce_GetExpectedResult()\n{\n \/\/arrange\n var func = Closure.GetFuncWithoutClosure();\n\n \/\/act\n var result = func(10);\n var result2 = func(10);\n \n \/\/assert\n Assert.Equal(2, result);\n Assert.Equal(2, result2);\n}<\/code><\/pre>\n\n\n\nWe have four tests:GetFuncWithClosure_InvokeOnce_GetExpectedResult<\/code>, GetFuncWithClosure_InvokeMoreThanOnce_GetUnexpectedResult<\/code>, GetFuncWithoutClosure_InvokeOnce_GetExpectedResult<\/code>, GetFuncWithoutClosure_InvokeMoreThanOnce_GetExpectedResult<\/code>.<\/p>\n\n\n\nFirst two tests test GetFuncWithClosure<\/code>, whereas next two test GetFuncWithoutClosure<\/code> method. Surprisingly, GetFuncWithClosure<\/code>throws the DivideByZeroException exception, when GetFuncWithClosure<\/code> invoked second time.<\/p>\n\n\n\nTo get better insights we should go to the sharplab.io<\/a>.<\/p>\n\n\n\nBehind the scenes<\/h1>\n\n\n\n
If we paste our code in sharplab.io<\/a>, we will get the answer of the DivideByZeroException exception occurrence cause. So, let’s take a closer look at the following:<\/p>\n\n\n\n![\"\"](\"https:\/\/baldsolutions.com\/wp-content\/uploads\/2022\/03\/image-6.png\")
<\/figure>\n\n\n\nFor now let’s focus on the problematic GetFuncWithClosure<\/code>method. As you can see the method body uses now some strange <>c__DisplayClass0_0<\/code> class. Well, that is the exact example of the closure in C#. Compiler had to create something (<>c__DisplayClass0_0<\/code> class) to store the int internalVariable<\/code> just because the variable has been used inside the returned Func<int>. On the other hand, GetFuncWithoutClosure<\/code>method doesn’t use any external data within returned Func<int, int>, therefore no corresponding *DisplayClass*.<\/p>\n\n\n\nOnce we know that, it’s obvious why the DivideByZeroException exception occurs. First invocation (i.e. func()<\/code>) subtracts 5 from the <>c__DisplayClass0_0<\/code> object’s internalVariable<\/code>. So far so good, but second invocation subtracts 5 again from the <>c__DisplayClass0_0<\/code> object’s internalVariable<\/code> zeroing its value. One thing is certain: Math does not like to divide by zero.<\/p>\n\n\n\nSummary<\/h1>\n\n\n\n
To sum up, closure is an interesting part of computer programming that definitely deserves to be understood. The above example shows that the code might work in an unexpected manner, until we don’t know what happens behind the scenes. To get better understanding what the closure is I highly recommend to read some articles\/blogs (including aforementioned Wikipedia <\/a>page) or watch some YouTube video (e.g. this<\/a> one).<\/p>\n\n\n\nAs always I’ve prepared an example and you can find it on my github<\/a>, project: closure<\/a>, test project: closure-tests.<\/a> I encourage you to go through the code, debug it and try to test your own scenarios.<\/p>\n\n\n\nHave a nice day, bye!<\/p>\n","protected":false},"excerpt":{"rendered":"
Introduction Closure itself isn’t a brand new topic. I’ve seen many blog articles that explain the topic (there’s even a Wikipedia page in a few…<\/p>\n