長年VB6のプログラマをやっていた技術者がVB.NETでプログラムを作る時によく悩むのが
APIの宣言方法と引数の渡し方だと思います。特にVB6でAny、String、構造体を使っていた時に
VB.NETではどうするのか、、、長時間、検索エンジンに頼る事になると思います。
(VB.NETでAPIなんか使うな！！ とか言わないの・・・)

VB6とVB.NETのDeclare(宣言)方法の違いのポイントは以下のような感じです。
(１)VB6のDeclareはVB.NETのDeclareに対応し同じなのです。
　　このサンプルではDeclareを使用していますが、<DllImport("mydll.dll")>を使用した方が
　　良いかもしれません。
(２)VB6のLongは4バイトなのでVB.NETのIntegerは4バイト(Longは8バイト)に対応します。
(３)VB6の"String * 10"のような変数の宣言はVB.NETの"New String("", 10)"に対応します。
　　ただし、構造体のメンバ変数の場合は(３)参照。
(４)構造体のメンバ変数が配列の場合の対応は以下のようになります。(サンプル有り)
　　・文字列の場合
   　 (VB6   の場合)s2 As String * 7
　　　(VB.NETの場合)<MarshalAs(UnmanagedType.ByValTStr, SizeConst:=7)> Dim s2 As String
　　・配列の場合
 　   (VB6   の場合)s4(1) As Long
　　　(VB.NETの場合)<MarshalAs(UnmanagedType.ByValArray, SizeConst:=2)> Dim s4 As Integer()
(５)VB.NETにはAnyのような曖昧な宣言は出来ないので、最適な型に置き換えます。
(６)VB6のVariantはVB.NETのObjectに対応します。

サンプルソースを見た方が速いと思いますので、
VB6でのDeclare(宣言)方法とVB.NETでのDeclare(宣言)方法を比較したサンプルソースを以下に載せます。

このVB.NETのサンプルをC#.NETに移植したものがあります。
移植後のサンプルはVB6とC#.NETのAPI宣言(Declare)の違いを比較したサンプルです。
VB6とVB.NETとC#.NETのAPI宣言の比較が出来ます。

ちなみに、今回使用したAPIはVC++で作ったサンプルで、第一引数を第二引数にコピーする仕様です。
戻り値は第一引数のサイズを返します。VC++でDLLを作成するサンプルはこちらです。
VCからもVBからも呼び出せるDLLのサンプル(VC6)



＜比較結果詳細＞
１．VC++で作ったAPIのサンプルソース
typedef struct {
	char s1;
	char s2[7];
	int  s3;
	int  s4[2];
} MYSTRUCT;

__declspec(dllexport) int __stdcall MyCopyChar(char v1, char *v2){
	*v2 = v1;
    return sizeof(v1);
}

__declspec(dllexport) int __stdcall MyCopyString(char *v1, char *v2){
	strcpy(v2, v1);
    return strlen(v1);
}

__declspec(dllexport) int __stdcall MyCopyInteger(int v1, char *v2){
	*v2 = v1;
    return sizeof(v1);
}

__declspec(dllexport) int __stdcall MyCopyStruct(MYSTRUCT *v1, MYSTRUCT *v2){
	v2->s1 = v1->s1;
	memcpy(v2->s2, v1->s2, sizeof(v1->s2));
	v2->s3 = v1->s3;
	memcpy(v2->s4, v1->s4, sizeof(v1->s4));
    return sizeof(*v1);
}


２．VB6からVC++のAPIを呼ぶサンプルソース
Private Type MYSTRUCT
    s1 As Byte
    s2 As String * 7
    s3 As Long
    s4(1) As Long
End Type

Private Declare Function MyCopyChar Lib "mydll.dll" _
    (ByVal v1 As Byte, ByRef v2 As Byte) As Long
Private Declare Function MyCopyString Lib "mydll.dll" _
    (ByVal v1 As String, ByVal v2 As String) As Long
Private Declare Function MyCopyInteger Lib "mydll.dll" _
    (ByVal v1 As Long, ByRef v2 As Long) As Long
Private Declare Function MyCopyStruct Lib "mydll.dll" _
    (ByRef v1 As MYSTRUCT, ByRef v2 As MYSTRUCT) As Long

Private Sub Command1_Click()
    Dim rtn As Long
    
    ' APIの引数としてByteの値とByteの参照(アドレス)を渡す例
    Dim v1_b As Byte
    Dim v2_b As Byte
    v1_b = 65
    rtn = MyCopyChar(v1_b, v2_b)
    Debug.Print ("------MyCopyChar------")
    Debug.Print "v1_b=" & v1_b & ":" & "v2_b=" & v2_b
    Debug.Print "rtn=" & rtn
    
    ' APIの引数としてStringの値とStringの参照(アドレス)を渡す例
    Dim v1_s As String
    Dim v2_s As String * 10
    v1_s = "abcdefg"
    rtn = MyCopyString(v1_s, v2_s)
    Debug.Print ("------MyCopyString------")
    Debug.Print "v1_s=" & v1_s & ":" & "v2_s=" & v2_s
    Debug.Print "rtn=" & rtn

    ' APIの引数としてLongの値とLongの参照(アドレス)を渡す例
    Dim v1_l As Long
    Dim v2_l As Long
    v1_l = 65
    rtn = MyCopyInteger(v1_l, v2_l)
    Debug.Print ("------MyCopyInteger------")
    Debug.Print "v1_l=" & v1_l & ":" & "v2_l=" & v2_l
    Debug.Print "rtn=" & rtn
    
    ' APIの引数として構造体(配列メンバ有り)の参照(アドレス)を渡す例
    Dim v1_t As MYSTRUCT
    Dim v2_t As MYSTRUCT
    v1_t.s1 = 65
    v1_t.s2 = "abcdefg123"
    v1_t.s3 = 66
    v1_t.s4(0) = 67
    v1_t.s4(1) = 68
    rtn = MyCopyStruct(v1_t, v2_t)
    Debug.Print ("------MyCopyStruct------")
    Debug.Print "v1_t.s1=" & v1_t.s1 & ":" & "v2_t.s1=" & v2_t.s1
    Debug.Print "v1_t.s2=" & v1_t.s2 & ":" & "v2_t.s2=" & v2_t.s2
    Debug.Print "v1_t.s3=" & v1_t.s3 & ":" & "v2_t.s3=" & v2_t.s3
    Debug.Print "v1_t.s4(0)=" & v1_t.s4(0) & ":" & "v2_t.s4(0)=" & v2_t.s4(0)
    Debug.Print "v1_t.s4(1)=" & v1_t.s4(1) & ":" & "v2_t.s4(1)=" & v2_t.s4(1)
    Debug.Print "rtn=" & rtn

End Sub

＝＝＝実行結果＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝
------MyCopyChar------
v1_b=65:v2_b=65
rtn=1
------MyCopyString------
v1_s=abcdefg:v2_s=abcdefg   
rtn=7
------MyCopyInteger------
v1_l=65:v2_l=65
rtn=4
------MyCopyStruct------
v1_t.s1=65:v2_t.s1=65
v1_t.s2=abcdefg:v2_t.s2=abcdefg
v1_t.s3=66:v2_t.s3=66
v1_t.s4(0)=67:v2_t.s4(0)=67
v1_t.s4(1)=68:v2_t.s4(1)=68
rtn=20


３．VB.NET(2005)からVC++のAPIを呼ぶサンプルソース
Imports System.Runtime.InteropServices
Public Class Form1

    Private Structure MYSTRUCT1
        Dim s1 As Byte
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=7)> Dim s2 As String
        Dim s3 As Integer
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=2)> Dim s4 As Integer()
    End Structure

    Private Structure MYSTRUCT2
        Dim s1 As Byte
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=7)> Dim s2 As Byte()
        Dim s3 As Integer
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=2)> Dim s4 As Integer()
    End Structure

    Private Structure MYSTRUCT3
        Dim s1 As Byte
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=7)> Dim s2 As Byte()
        Dim s3 As Integer
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=2)> Dim s4 As Integer()
        Sub New(ByVal dummy As Integer)
            ReDim s2(6)
            ReDim s4(1)
        End Sub
    End Structure

    Private Declare Function MyCopyChar Lib "mydll.dll" _
        (ByVal v1 As Byte, ByRef v2 As Byte) As Integer
    Private Declare Function MyCopyString Lib "mydll.dll" _
        (ByVal v1 As String, ByVal v2 As String) As Integer
    Private Declare Function MyCopyString Lib "mydll.dll" _
        (ByVal v1 As String, ByVal v2 As Byte()) As Integer
    Private Declare Function MyCopyInteger Lib "mydll.dll" _
        (ByVal v1 As Integer, ByRef v2 As Integer) As Integer
    Private Declare Function MyCopyStruct Lib "mydll.dll" _
        (ByRef v1 As MYSTRUCT1, ByRef v2 As MYSTRUCT1) As Integer
    Private Declare Function MyCopyStruct Lib "mydll.dll" _
        (ByRef v1 As MYSTRUCT2, ByRef v2 As MYSTRUCT2) As Integer
    Private Declare Function MyCopyStruct Lib "mydll.dll" _
        (ByRef v1 As MYSTRUCT3, ByRef v2 As MYSTRUCT3) As Integer

    Private Sub Button1_Click( _
        ByVal sender As System.Object, ByVal e As System.EventArgs) _
        Handles Button1.Click
        Dim rtn As Integer

        ' APIの引数としてByteの値とByteの参照(アドレス)を渡す例
        Dim v1_b As Byte
        Dim v2_b As Byte
        v1_b = 65
        rtn = MyCopyChar(v1_b, v2_b)
        Debug.Print("------MyCopyChar------")
        Debug.Print("v1_b=" & v1_b & ":" & "v2_b=" & v2_b)
        Debug.Print("rtn=" & rtn)

        ' APIの引数としてStringの値とStringの参照(アドレス)を渡す例１
        Dim v1_s1 As String
        Dim v2_s1 As String = New String("", 10)
        v1_s1 = "abcdefg"
        rtn = MyCopyString(v1_s1, v2_s1)
        Dim v2_s1_o As String = v2_s1
        Dim l1 As Integer = v2_s1_o.IndexOf(Chr(0))
        If l1 >= 0 Then v2_s1_o = v2_s1_o.Substring(0, l1)
        Debug.Print("------MyCopyString1------")
        Debug.Print("v1_s1=" & v1_s1 & ":" & "v2_s1=" & v2_s1_o)
        Debug.Print("rtn=" & rtn)

        ' APIの引数としてStringの値とStringの参照(アドレス)を渡す例２
        Dim v1_s2 As String
        Dim v2_s2 As Byte() = New Byte(9) {}
        v1_s2 = "abcdefg"
        rtn = MyCopyString(v1_s2, v2_s2)
        Dim v2_s2_o As String = _
            System.Text.Encoding.GetEncoding("SHIFT-JIS").GetString(v2_s2)
        Dim l2 As Integer = v2_s2_o.IndexOf(Chr(0))
        If l2 >= 0 Then v2_s2_o = v2_s2_o.Substring(0, l2)
        Debug.Print("------MyCopyString2------")
        Debug.Print("v1_s2=" & v1_s1 & ":" & "v2_s2=" & v2_s2_o)
        Debug.Print("rtn=" & rtn)

        ' APIの引数としてIntegerの値とIntegerの参照(アドレス)を渡す例
        Dim v1_l As Integer
        Dim v2_l As Integer
        v1_l = 65
        rtn = MyCopyInteger(v1_l, v2_l)
        Debug.Print("------MyCopyInteger------")
        Debug.Print("v1_l=" & v1_l & ":" & "v2_l=" & v2_l)
        Debug.Print("rtn=" & rtn)

        ' APIの引数として構造体(配列メンバ有り)の参照(アドレス)を渡す例１
        Dim v1_t1 As MYSTRUCT1 = New MYSTRUCT1
        Dim v2_t1 As MYSTRUCT1 = New MYSTRUCT1
        v1_t1.s1 = 65
        v1_t1.s2 = "abcdefg123"
        v1_t1.s3 = 66
        v1_t1.s4 = New Integer(1) {67, 68}
        rtn = MyCopyStruct(v1_t1, v2_t1)
        Debug.Print("------MyCopyStruct1------")
        Debug.Print("v1_t1.s1=" & v1_t1.s1 & ":" & "v2_t1.s1=" & v2_t1.s1)
        Debug.Print("v1_t1.s2=" & v1_t1.s2 & ":" & "v2_t1.s2=" & v2_t1.s2)
        Debug.Print("v1_t1.s3=" & v1_t1.s3 & ":" & "v2_t1.s3=" & v2_t1.s3)
        Debug.Print("v1_t1.s4(0)=" & v1_t1.s4(0) & ":" & "v2_t1.s4(0)=" & v2_t1.s4(0))
        Debug.Print("v1_t1.s4(1)=" & v1_t1.s4(1) & ":" & "v2_t1.s4(1)=" & v2_t1.s4(1))
        Debug.Print("rtn=" & rtn)

        ' APIの引数として構造体(配列メンバ有り)の参照(アドレス)を渡す例２
        Dim v1_t2 As MYSTRUCT2 = New MYSTRUCT2
        Dim v2_t2 As MYSTRUCT2 = New MYSTRUCT2
        v1_t2.s1 = 65
        v1_t2.s2 = System.Text.Encoding.GetEncoding("SHIFT-JIS").GetBytes("abcdefg123")
        v1_t2.s3 = 66
        v1_t2.s4 = New Integer(1) {67, 68}
        rtn = MyCopyStruct(v1_t2, v2_t2)
        Debug.Print("------MyCopyStruct2------")
        Debug.Print("v1_t2.s1=" & v1_t2.s1 & ":" & "v2_t2.s1=" & v2_t2.s1)
        Debug.Print("v1_t2.s2=" & _
            System.Text.Encoding.GetEncoding("SHIFT-JIS").GetString(v1_t2.s2) & _
            ":" & "v2_t.s2=" & _
            System.Text.Encoding.GetEncoding("SHIFT-JIS").GetString(v2_t2.s2))
        Debug.Print("v1_t2.s3=" & v1_t2.s3 & ":" & "v2_t2.s3=" & v2_t2.s3)
        Debug.Print("v1_t2.s4(0)=" & v1_t2.s4(0) & ":" & "v2_t.s4(0)=" & v2_t2.s4(0))
        Debug.Print("v1_t2.s4(1)=" & v1_t2.s4(1) & ":" & "v2_t.s4(1)=" & v2_t2.s4(1))
        Debug.Print("rtn=" & rtn)

        ' APIの引数として構造体(配列メンバ有り)の参照(アドレス)を渡す例３
        Dim v1_t3 As MYSTRUCT3 = New MYSTRUCT3(0)
        Dim v2_t3 As MYSTRUCT3 = New MYSTRUCT3(0)
        v1_t3.s1 = 65
        v1_t3.s2(0) = Asc("a")
        v1_t3.s2(1) = Asc("b")
        v1_t3.s2(2) = Asc("c")
        v1_t3.s2(3) = Asc("d")
        v1_t3.s2(4) = Asc("e")
        v1_t3.s2(5) = Asc("f")
        v1_t3.s2(6) = Asc("g")
        v1_t3.s3 = 66
        v1_t3.s4(0) = 67
        v1_t3.s4(1) = 68
        rtn = MyCopyStruct(v1_t3, v2_t3)
        Debug.Print("------MyCopyStruct3------")
        Debug.Print("v1_t3.s1=" & v1_t3.s1 & ":" & "v2_t3.s1=" & v2_t3.s1)
        Debug.Print("v1_t3.s2=" & _
            System.Text.Encoding.GetEncoding("SHIFT-JIS").GetString(v1_t3.s2) & _
            ":" & "v2_t3.s2=" & _
            System.Text.Encoding.GetEncoding("SHIFT-JIS").GetString(v2_t3.s2))
        Debug.Print("v1_t3.s3=" & v1_t3.s3 & ":" & "v2_t3.s3=" & v2_t3.s3)
        Debug.Print("v1_t3.s4(0)=" & v1_t3.s4(0) & ":" & "v2_t3.s4(0)=" & v2_t3.s4(0))
        Debug.Print("v1_t3.s4(1)=" & v1_t3.s4(1) & ":" & "v2_t3.s4(1)=" & v2_t3.s4(1))
        Debug.Print("rtn=" & rtn)
    End Sub
End Class

＝＝＝実行結果＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝
------MyCopyChar------
v1_b=65:v2_b=65
rtn=1
------MyCopyString1------
v1_s1=abcdefg:v2_s1=abcdefg
rtn=7
------MyCopyString2------
v1_s2=abcdefg:v2_s2=abcdefg
rtn=7
------MyCopyInteger------
v1_l=65:v2_l=65
rtn=4
------MyCopyStruct1------
v1_t1.s1=65:v2_t1.s1=65
v1_t1.s2=abcdefg:v2_t1.s2=abcdefg
v1_t1.s3=66:v2_t1.s3=66
v1_t1.s4(0)=67:v2_t1.s4(0)=67
v1_t1.s4(1)=68:v2_t1.s4(1)=68
rtn=20
------MyCopyStruct2------
v1_t2.s1=65:v2_t2.s1=65
v1_t2.s2=abcdefg:v2_t.s2=abcdefg
v1_t2.s3=66:v2_t2.s3=66
v1_t2.s4(0)=67:v2_t.s4(0)=67
v1_t2.s4(1)=68:v2_t.s4(1)=68
rtn=20
------MyCopyStruct3------
v1_t3.s1=65:v2_t3.s1=65
v1_t3.s2=abcdefg:v2_t3.s2=abcdefg
v1_t3.s3=66:v2_t3.s3=66
v1_t3.s4(0)=67:v2_t3.s4(0)=67
v1_t3.s4(1)=68:v2_t3.s4(1)=68
rtn=20