Marshalling Different Types of Arrays

Article
02/09/2023

An array is a reference type in managed code that contains one or more elements of the same type. Although arrays are reference types, they are passed as In parameters to unmanaged functions. This behavior is inconsistent with the way managed arrays are passed to managed objects, which is as In/Out parameters. For additional details, see Copying and Pinning.

The following table lists marshalling options for arrays and describes their usage.

Array	Description
Of integers by value.	Passes an array of integers as an In parameter.
Of integers by reference.	Passes an array of integers as an In/Out parameter.
Of integers by value (two-dimensional).	Passes a matrix of integers as an In parameter.
Of strings by value.	Passes an array of strings as an In parameter.
Of structures with integers.	Passes an array of structures that contain integers as an In parameter.
Of structures with strings.	Passes an array of structures that contain only strings as an In/Out parameter. Members of the array can be changed.

Example

This sample demonstrates how to pass the following types of arrays:

Array of integers by value.
Array of integers by reference, which can be resized.
Multidimensional array (matrix) of integers by value.
Array of strings by value.
Array of structures with integers.
Array of structures with strings.

Unless an array is explicitly marshalled by reference, the default behavior marshals the array as an In parameter. You can change this behavior by applying the InAttribute and OutAttribute attributes explicitly.

The Arrays sample uses the following unmanaged functions, shown with their original function declaration:

TestArrayOfInts exported from PinvokeLib.dll.

int TestArrayOfInts(int* pArray, int pSize);

TestRefArrayOfInts exported from PinvokeLib.dll.

int TestRefArrayOfInts(int** ppArray, int* pSize);

TestMatrixOfInts exported from PinvokeLib.dll.

int TestMatrixOfInts(int pMatrix[][COL_DIM], int row);

TestArrayOfStrings exported from PinvokeLib.dll.

int TestArrayOfStrings(char** ppStrArray, int size);

TestArrayOfStructs exported from PinvokeLib.dll.

int TestArrayOfStructs(MYPOINT* pPointArray, int size);

TestArrayOfStructs2 exported from PinvokeLib.dll.

int TestArrayOfStructs2 (MYPERSON* pPersonArray, int size);

PinvokeLib.dll is a custom unmanaged library that contains implementations for the previously listed functions and two structure variables, MYPOINT and MYPERSON. The structures contain the following elements:

typedef struct _MYPOINT  
{  
   int x;
   int y;
} MYPOINT;  
  
typedef struct _MYPERSON  
{  
   char* first;
   char* last;
} MYPERSON;

In this sample, the MyPoint and MyPerson structures contain embedded types. The StructLayoutAttribute attribute is set to ensure that the members are arranged in memory sequentially, in the order in which they appear.

The NativeMethods class contains a set of methods called by the App class. For specific details about passing arrays, see the comments in the following sample. An array, which is a reference type, is passed as an In parameter by default. For the caller to receive the results, InAttribute and OutAttribute must be applied explicitly to the argument containing the array.

Declaring Prototypes

// Declares a managed structure for each unmanaged structure.
[StructLayout(LayoutKind.Sequential)]
public struct MyPoint
{
    public int X;
    public int Y;

    public MyPoint(int x, int y)
    {
        this.X = x;
        this.Y = y;
    }
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyPerson
{
    public string First;
    public string Last;

    public MyPerson(string first, string last)
    {
        this.First = first;
        this.Last = last;
    }
}

internal static class NativeMethods
{
    // Declares a managed prototype for an array of integers by value.
    // The array size cannot be changed, but the array is copied back.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestArrayOfInts(
        [In, Out] int[] array, int size);

    // Declares a managed prototype for an array of integers by reference.
    // The array size can change, but the array is not copied back
    // automatically because the marshaler does not know the resulting size.
    // The copy must be performed manually.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestRefArrayOfInts(
        ref IntPtr array, ref int size);

    // Declares a managed prototype for a matrix of integers by value.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestMatrixOfInts(
        [In, Out] int[,] pMatrix, int row);

    // Declares a managed prototype for an array of strings by value.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestArrayOfstrings(
        [In, Out] string[] stringArray, int size);

    // Declares a managed prototype for an array of structures with integers.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestArrayOfStructs(
        [In, Out] MyPoint[] pointArray, int size);

    // Declares a managed prototype for an array of structures with strings.
    [DllImport("..\\LIB\\PinvokeLib.dll", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int TestArrayOfStructs2(
        [In, Out] MyPerson[] personArray, int size);
}

' Declares a managed structure for each unmanaged structure.
<StructLayout(LayoutKind.Sequential)>
Public Structure MyPoint
    Public x As Integer
    Public y As Integer
    Public Sub New(x As Integer, y As Integer)
        Me.x = x
        Me.y = y
    End Sub
End Structure

<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)>
Public Structure MyPerson
    Public first As String
    Public last As String
    Public Sub New(first As String, last As String)
        Me.first = first
        Me.last = last
    End Sub
End Structure

Friend Class NativeMethods
    ' Declares a managed prototype for an array of integers by value.
    ' The array size cannot be changed, but the array is copied back.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestArrayOfInts(
        <[In], Out> ByVal myArray() As Integer, ByVal size As Integer) _
        As Integer
    End Function

    ' Declares managed prototype for an array of integers by reference.
    ' The array size can change, but the array is not copied back 
    ' automatically because the marshaler does not know the resulting size.
    ' The copy must be performed manually.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestRefArrayOfInts(
        ByRef myArray As IntPtr, ByRef size As Integer) As Integer
    End Function

    ' Declares a managed prototype for a matrix of integers by value.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestMatrixOfInts(
        <[In], Out> ByVal matrix(,) As Integer, ByVal row As Integer) _
        As Integer
    End Function

    ' Declares a managed prototype for an array of strings by value.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestArrayOfStrings(
        <[In], Out> ByVal strArray() As String, ByVal size As Integer) _
        As Integer
    End Function

    ' Declares a managed prototype for an array of structures with 
    ' integers.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestArrayOfStructs(
        <[In], Out> ByVal pointArray() As MyPoint, ByVal size As Integer) _
        As Integer
    End Function

    ' Declares a managed prototype for an array of structures with strings.
    <DllImport("..\LIB\PinvokeLib.dll", CallingConvention:=CallingConvention.Cdecl)>
    Friend Shared Function TestArrayOfStructs2(
        <[In], Out> ByVal personArray() As MyPerson, ByVal size As Integer) _
        As Integer
    End Function
End Class

Calling Functions

public class App
{
    public static void Main()
    {
        // array ByVal
        int[] array1 = new int[10];
        Console.WriteLine("Integer array passed ByVal before call:");
        for (int i = 0; i < array1.Length; i++)
        {
            array1[i] = i;
            Console.Write(" " + array1[i]);
        }

        int sum1 = NativeMethods.TestArrayOfInts(array1, array1.Length);
        Console.WriteLine("\nSum of elements:" + sum1);
        Console.WriteLine("\nInteger array passed ByVal after call:");

        foreach (int i in array1)
        {
            Console.Write(" " + i);
        }

        // array ByRef
        int[] array2 = new int[10];
        int size = array2.Length;
        Console.WriteLine("\n\nInteger array passed ByRef before call:");
        for (int i = 0; i < array2.Length; i++)
        {
            array2[i] = i;
            Console.Write(" " + array2[i]);
        }

        IntPtr buffer = Marshal.AllocCoTaskMem(Marshal.SizeOf(size)
           * array2.Length);
        Marshal.Copy(array2, 0, buffer, array2.Length);

        int sum2 = NativeMethods.TestRefArrayOfInts(ref buffer, ref size);
        Console.WriteLine("\nSum of elements:" + sum2);
        if (size > 0)
        {
            int[] arrayRes = new int[size];
            Marshal.Copy(buffer, arrayRes, 0, size);
            Marshal.FreeCoTaskMem(buffer);
            Console.WriteLine("\nInteger array passed ByRef after call:");
            foreach (int i in arrayRes)
            {
                Console.Write(" " + i);
            }
        }
        else
        {
            Console.WriteLine("\nArray after call is empty");
        }

        // matrix ByVal
        const int DIM = 5;
        int[,] matrix = new int[DIM, DIM];

        Console.WriteLine("\n\nMatrix before call:");
        for (int i = 0; i < DIM; i++)
        {
            for (int j = 0; j < DIM; j++)
            {
                matrix[i, j] = j;
                Console.Write(" " + matrix[i, j]);
            }

            Console.WriteLine("");
        }

        int sum3 = NativeMethods.TestMatrixOfInts(matrix, DIM);
        Console.WriteLine("\nSum of elements:" + sum3);
        Console.WriteLine("\nMatrix after call:");
        for (int i = 0; i < DIM; i++)
        {
            for (int j = 0; j < DIM; j++)
            {
                Console.Write(" " + matrix[i, j]);
            }

            Console.WriteLine("");
        }

        // string array ByVal
        string[] strArray = { "one", "two", "three", "four", "five" };
        Console.WriteLine("\n\nstring array before call:");
        foreach (string s in strArray)
        {
            Console.Write(" " + s);
        }

        int lenSum = NativeMethods.TestArrayOfstrings(strArray, strArray.Length);
        Console.WriteLine("\nSum of string lengths:" + lenSum);
        Console.WriteLine("\nstring array after call:");
        foreach (string s in strArray)
        {
            Console.Write(" " + s);
        }

        // struct array ByVal
        MyPoint[] points = { new MyPoint(1, 1), new MyPoint(2, 2), new MyPoint(3, 3) };
        Console.WriteLine("\n\nPoints array before call:");
        foreach (MyPoint p in points)
        {
            Console.WriteLine($"X = {p.X}, Y = {p.Y}");
        }

        int allSum = NativeMethods.TestArrayOfStructs(points, points.Length);
        Console.WriteLine("\nSum of points:" + allSum);
        Console.WriteLine("\nPoints array after call:");
        foreach (MyPoint p in points)
        {
            Console.WriteLine($"X = {p.X}, Y = {p.Y}");
        }

        // struct with strings array ByVal
        MyPerson[] persons =
        {
            new MyPerson("Kim", "Akers"),
            new MyPerson("Adam", "Barr"),
            new MyPerson("Jo", "Brown")
        };

        Console.WriteLine("\n\nPersons array before call:");
        foreach (MyPerson pe in persons)
        {
            Console.WriteLine($"First = {pe.First}, Last = {pe.Last}");
        }

        int namesSum = NativeMethods.TestArrayOfStructs2(persons, persons.Length);
        Console.WriteLine("\nSum of name lengths:" + namesSum);
        Console.WriteLine("\n\nPersons array after call:");
        foreach (MyPerson pe in persons)
        {
            Console.WriteLine($"First = {pe.First}, Last = {pe.Last}");
        }
    }
}

Public Class App
    Public Shared Sub Main()
        ' array ByVal
        Dim array1(9) As Integer

        Console.WriteLine("Integer array passed ByVal before call:")
        Dim i As Integer
        For i = 0 To array1.Length - 1
            array1(i) = i
            Console.Write(" " & array1(i))
        Next i

        Dim sum1 As Integer = NativeMethods.TestArrayOfInts(array1, array1.Length)
        Console.WriteLine(ControlChars.CrLf & "Sum of elements:" & sum1)
        Console.WriteLine(ControlChars.CrLf & "Integer array passed ByVal after call:")
        For Each i In array1
            Console.Write(" " & i)
        Next i

        ' array ByRef
        Dim array2(9) As Integer
        Dim arraySize As Integer = array2.Length
        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf &
            "Integer array passed ByRef before call:")
        For i = 0 To array2.Length - 1
            array2(i) = i
            Console.Write(" " & array2(i))
        Next i
        Dim buffer As IntPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf(
            arraySize) * array2.Length)
        Marshal.Copy(array2, 0, buffer, array2.Length)
        Dim sum2 As Integer = NativeMethods.TestRefArrayOfInts(buffer,
            arraySize)
        Console.WriteLine(ControlChars.CrLf & "Sum of elements:" & sum2)

        If arraySize > 0 Then
            Dim arrayRes(arraySize - 1) As Integer
            Marshal.Copy(buffer, arrayRes, 0, arraySize)
            Marshal.FreeCoTaskMem(buffer)

            Console.WriteLine(ControlChars.CrLf & "Integer array passed ByRef after call:")
            For Each i In arrayRes
                Console.Write(" " & i)
            Next i
        Else
            Console.WriteLine(ControlChars.CrLf & "Array after call is empty")
        End If

        ' matrix ByVal 
        Const [DIM] As Integer = 4
        Dim matrix([DIM], [DIM]) As Integer

        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf &
            "Matrix before call:")
        For i = 0 To [DIM]
            Dim j As Integer
            For j = 0 To [DIM]
                matrix(i, j) = j
                Console.Write(" " & matrix(i, j))
            Next j
            Console.WriteLine("")
        Next i

        Dim sum3 As Integer = NativeMethods.TestMatrixOfInts(matrix, [DIM] + 1)
        Console.WriteLine(ControlChars.CrLf & "Sum of elements:" & sum3)
        Console.WriteLine(ControlChars.CrLf & "Matrix after call:")
        For i = 0 To [DIM]
            Dim j As Integer
            For j = 0 To [DIM]
                Console.Write(" " & matrix(i, j))
            Next j
            Console.WriteLine("")
        Next i

        ' string array ByVal 
        Dim strArray As String() = {"one", "two", "three", "four",
            "five"}
        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf &
            "String array before call:")
        Dim s As String
        For Each s In strArray
            Console.Write(" " & s)
        Next s
        Dim lenSum As Integer = NativeMethods.TestArrayOfStrings(
            strArray, strArray.Length)
        Console.WriteLine(ControlChars.CrLf &
            "Sum of string lengths:" & lenSum)
        Console.WriteLine(ControlChars.CrLf & "String array after call:")
        For Each s In strArray
            Console.Write(" " & s)
        Next s

        ' struct array ByVal 
        Dim points As MyPoint() = {New MyPoint(1, 1), New MyPoint(2, 2),
            New MyPoint(3, 3)}
        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf &
            "Points array before call:")
        Dim p As MyPoint
        For Each p In points
            Console.WriteLine($"x = {p.x}, y = {p.y}")
        Next p
        Dim allSum As Integer = NativeMethods.TestArrayOfStructs(points,
            points.Length)
        Console.WriteLine(ControlChars.CrLf & "Sum of points:" & allSum)
        Console.WriteLine(ControlChars.CrLf & "Points array after call:")
        For Each p In points
            Console.WriteLine($"x = {p.x}, y = {p.y}")
        Next p

        ' struct with strings array ByVal 
        Dim persons As MyPerson() = {New MyPerson("Kim", "Akers"),
            New MyPerson("Adam", "Barr"),
            New MyPerson("Jo", "Brown")}
        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf &
            "Persons array before call:")
        Dim pe As MyPerson
        For Each pe In persons
            Console.WriteLine($"first = {pe.first}, last = {pe.last}")
        Next pe

        Dim namesSum As Integer = NativeMethods.TestArrayOfStructs2(persons,
            persons.Length)
        Console.WriteLine(ControlChars.CrLf & "Sum of name lengths:" &
            namesSum)
        Console.WriteLine(ControlChars.CrLf & ControlChars.CrLf _
            & "Persons array after call:")
        For Each pe In persons
            Console.WriteLine($"first = {pe.first}, last = {pe.last}")
        Next pe
    End Sub
End Class

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Marshalling Different Types of Arrays

Example

Declaring Prototypes

Calling Functions

See also

Additional resources