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sum += a[x, y]; } } return sum; } private static Int32 SafeJaggedArrayAccess(Int32[][] a) { Int32 sum = 0; for (Int32 x = 0; x < c_numElements; x++) { for (Int32 y = 0; y < c_numElements; y++) { sum += a[x][y]; } } return sum; } private static unsafe Int32 Unsafe2DimArrayAccess(Int32[,] a) { Int32 sum = 0; fixed (Int32* pi = a) { for (Int32 x = 0; x < c_numElements; x++) { Int32 baseOfDim = x * c_numElements; for (Int32 y = 0; y < c_numElements; y++) { sum += pi[baseOfDim + y]; } } } return sum; } }
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The Unsafe2DimArrayAccess method is marked with the unsafe modifier, which is required to use C# s fixed statement . To compile this code, you ll have to specify the /unsafe switch when invoking the C# compiler or check the Allow Unsafe Code check box on the Build tab of the Project Properties pane in Microsoft Visual Studio . When I run this program on my machine, I get the following output:
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00:00:02.0017692: Safe2DimArrayAccess 00:00:01.5197844: SafeJaggedArrayAccess 00:00:01.7343436: Unsafe2DimArrayAccess
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As you can see, the safe two-dimensional array access technique is the slowest . The safe jagged array access technique takes a little less time to complete than the safe two-dimensional array access technique . However, you should note that creating the jagged array is more time-consuming than creating the multi-dimensional array because creating the jagged array requires an object to be allocated on the heap for each dimension, causing the garbage collector to kick in periodically . So there is a trade-off: If you need to create a lot of multidimensional arrays and you intend to access the elements infrequently, it is quicker to create a multi-dimensional array . If you need to create the multi-dimensional array just once, and you access its elements frequently, a jagged array will give you better performance . Certainly, in most applications, the latter scenario is more common .
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Finally, notice that the unsafe two-dimensional array access technique is about as fast as the safe two-dimensional array access technique, but it would be considered the fastest of them all if you also took into account that it accesses a single two-dimensional array (which is one memory allocation), as compared to creating the jagged array (which requires many memory allocations) . Obviously, the unsafe technique has a time and place when it can best be used by your own code, but beware that there are three serious downsides to using this technique:
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The code that manipulates the array elements is more complicated to read and write than that which manipulates the elements using the other techniques because you are using C# s fixed statement and performing memory-address calculations . If you make a mistake in the calculation, you are accessing memory that is not part of the array . This can result in an incorrect calculation, corruption of memory, a type-safety violation, and a potential security hole . Due to the potential problems, the CLR forbids unsafe code from running in reducedsecurity environments (like Microsoft Silverlight) .
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Unsafe array access is very powerful because it allows you to access:
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Elements within a managed array object that resides on the heap (as the previous section demonstrated) . Elements within an array that resides on an unmanaged heap . The SecureString example in 14, Chars, Strings, and Working with Text, demonstrated using unsafe array access on an array returned from calling the System.Runtime. InteropServices.Marshal class s SecureStringToCoTaskMemUnicode method . Elements within an array that resides on the thread s stack .
In cases in which performance is extremely critical, you could avoid allocating a managed array object on the heap and instead allocate the array on the thread s stack by using C# s stackalloc statement (which works a lot like C s alloca function) . The stackalloc statement can be used to create a single-dimensional, zero-based array of value type elements only, and the value type must not contain any reference type fields . Really, you should think of this as allocating a block of memory that you can manipulate by using unsafe pointers, and therefore, you cannot pass the address of this memory buffer to the vast majority of FCL methods . Of course, the stack-allocated memory (array) will automatically be freed when the method returns; this is where we get the performance improvement . Using this feature also requires you specify the /unsafe switch to the C# compiler . The StackallocDemo method in the code below shows an example of how to use C# s stackalloc statement:
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