Find out the size of a .NET object

Don't forget that the size of an actual object doesn't include the size of any objects it references.

The only things which are likely to end up on the large object heap are arrays and strings - other objects tends to be relatively small in themselves. Even an object with (say) 10 reference type variables (4 bytes each on x86) and 10 GUIDs (16 bytes each) is only going to take up about 208 bytes (there's a bit of overhead for the type reference and sync block).

Likewise when thinking about the size of an array, don't forget that if the element type is a reference type, then it's only the size of the references that count for the array itself. In other words, even if you've got an array with 20,000 elements, the size of the array object itself will only be just over 80K (on x86) even if it references a lot more data.

You are getting into an area of advanced .NET debugging. Start with John Robins debugging books.

Use WinDBG with Sos.dll (part of .NET distribution) and Sosex.dll extensions. With these tools you can really see what's happening when your application is running. You will find answers to your above mentioned questions.

(Another recommendation would be to install Shared Source CLI 2.0, aka. Rotor 2, to see what's going on under the hood.)

Gomes's method simplified:

1. Go to Visual Studio (2010) Project Properties* → Debug tab → Enable unmanaged code debugging.

2. Set the break point in your code, start debugging (F5).

3. Open Debug → Windows → Immediate Window.

4. Enter .load sos

5. Enter (replace myObject with the name of your object)

   ? String.Format("{0:x}",Integer.Parse(System.Runtime.InteropServices.GCHandle.InternalAddrOfPinnedObject(System.Runtime.InteropServices.GCHandle.Alloc(myObject).GetHandleValue()).ToString())

      

6. Use the result as the parameter of !ObjSize

See: SOS.DLL, object Address and Visual Studio debugger Introduction

Example (we are looking for object named tbl):

.load sos
extension C:\Windows\Microsoft.NET\Framework\v4.0.30319\sos.dll loaded
? string.Format("{0:x}",Integer.Parse(System.Runtime.InteropServices.GCHandle.InternalAddrOfPinnedObject(System.Runtime.InteropServices.GCHandle.Alloc(tbl).GetHandleValue()).ToString())-4)
"27ccb18"
!ObjSize 27ccb18
PDB symbol for clr.dll not loaded
sizeof(027ccb18) =       154504 (     0x25b88) bytes (System.Data.DataTable)

Unless it's a huge valuetype or instance type (i.e. many thousands of fields), the only types you need to worry about are large arrays, or strings. Of course, to figure out the size of an array, you need to know the element size.

.NET (currently) aligns types in much the same way that native compilers align types. Fundamental types have natural alignments that are usually the rounded-up integral power of two closest to their size:

Single, Int32, UInt32 - 4
IntPtr, UIntPtr, pointers, references  - 4 on 32-bit, 8 on 64-bit
Double, Int64, UInt64 - 8
Char, Int16, UInt16   - 2
Byte, SByte           - 1

When assembling a type, the compiler will make sure that all fields of any given type have their starting offset within the instance aligned to a boundary that matches that type - assuming that explicit layout isn't being used.

User-defined types themselves have an alignment, which is calculated as the highest alignment of any of their field types. The type's size is extended if necessary to make the size of the type aligned too.

But of course, all reference types are still only IntPtr.Size in size and alignment, so the size of reference type will not affect arrays of that type.

Note that the CLR may choose, at its discretion, to layout types differently than described above, maybe to increase cache locality or reduce padding required by alignment.

In cases like Dictionary<TKey, TValue>, you can hack getting the object size by binary serialization. Here is a sample code:

var dictionary = new DictionaryGenerator().GetSomeLargeDictionary();
var memoryStream = new System.IO.MemoryStream();
var binaryFormatter = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter();
binaryFormatter.Serialize(memoryStream, dictionary);
Console.WriteLine($"Size of dictionary: {memoryStream.Position} byte(s)");