why does "UInt64[] arr=new UInt64[UInt64.MaxValue];" throw exception?

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Why does following code throw exception "Arithmetic operation resulted in an overflow." ?

UInt64[] arr=new UInt64[UInt64.MaxValue];

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I guess because totally 8 * UInt64.MaxValue bytes is requested to be allocated, and this multiplication obviously overflows a 64-bit register.

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Because indexers only take Int32 values. You can do

UInt64[] arr=new UInt64[Int32.MaxValue];

But thats the limit.

EDIT: Technically you can index an array with value structures that can theoretically be higher than Int32.MaxValue (because you can index an array with a long or a uint for example), however, you will run into that runtime error when the value exceeds Int32.MaxValue.

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Because

a) all object are limited to 2GB in .NET
b) You don't have 64 PetaBytes of memory to spend

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According to Microsoft's documentation, with framework .NET 4.5 these limitations apply:

• The maximum number of elements in an array is UInt32.MaxValue.
• The maximum index in any single dimension is 2,147,483,591 (0x7FFFFFC7) for byte arrays and arrays of single-byte structures, and 2,146,435,071 (0X7FEFFFFF) for other types.
• The maximum size for strings and other non-array objects is unchanged