Find out the size of a .NET object

I'm trying to find out how much memory my objects take to see how many of them are ending up on the Large Object Heap (which is anything over 85,000 bytes).

Is it as simple as adding 4 for an int, 8 for a long, 4 (or 8 if you're on 64 bit) for any reference types etc for each object, or are there overheads for methods, properties, etc.?

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.

Please follow these steps to get the size of the object.

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

2. Go to the Visual Studio Debug menu → Options and Settings → Debugging → Symbols.

3. There, enable Microsoft Symbol Server and leave the default (symbols may start a download).

4. Set the breakpoint in your code, start debugging (F5).

5. Open Debug → Windows → Immediate Window.

6. Enter .load sos.dll (Son of Strike)

7. Enter !DumpHeap -type MyClass (the object you want to find the size of)

8. From the output, locate the address of the object, i.e. (00a8197c)

   Address MT Size 00a8197c 00955124 36

9. Next, !ObjSize 00a8197c

10. There you go → sizeof(00a8197c) = 12 (0x48) bytes (MyClass)

If you can - Serialize it!

Dim myObjectSize As Long

Dim ms As New IO.MemoryStream
Dim bf As New Runtime.Serialization.Formatters.Binary.BinaryFormatter()
bf.Serialize(ms, myObject)
myObjectSize = ms.Position

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()) > - 4)

      

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.

As an estimate (in 2017) you can debug into your application, set a breakpoint before your dictionary comes to life, take a "Memory Usage Snapshot" (Tab: Memory Usage under Diagnostic Tools), fill your dictionary and get another snapshot.

It is not exact, but it is a good guesstimate.

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)");