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WIP - Fix performance and memory allocation issues in ReadPdhMultiString. #5845

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WIP - Fix performance and memory allocation issues in ReadPdhMultiString. #5845

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18 changes: 12 additions & 6 deletions src/Microsoft.PowerShell.Commands.Diagnostics/PdhHelper.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
//

using System;
using System.Text;
using System.Diagnostics;
using System.Globalization;
using System.Runtime.InteropServices;
Expand Down Expand Up @@ -485,7 +486,7 @@ private void ReadPdhMultiString(ref IntPtr strNative, Int32 strSize, ref StringC
{
Debug.Assert(strSize >= 2);
int offset = 0;
string allSubstringsWithNulls = "";
StringBuilder sb = new StringBuilder(strSize);
while (offset <= ((strSize * sizeof(char)) - 4))
{
Int32 next4 = Marshal.ReadInt32(strNative, offset);
Expand All @@ -494,14 +495,19 @@ private void ReadPdhMultiString(ref IntPtr strNative, Int32 strSize, ref StringC
break;
}

allSubstringsWithNulls += (char)next4;
char nextChar = (char)next4;
if (nextChar == '\0')
{
strColl.Add(sb.ToString());
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@dantraMSFT dantraMSFT Jan 11, 2018

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Adding to strColl should occur outside the while loop. The updated logic causes the contents of sb to be dropped on the floor by the break statement when next4 == 0.

Essentially, replace the original allSubstringsWithNulls += (char)next4 statement with sb.Append(nextChar) and move the strColl.Add statement to the original location after the while loop.

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@jcarino jcarino Jan 12, 2018

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You're right, but adding to strColl only outside the while loop has some memory implications (has to alloc at least one extra string before Split). Can we just add another strColl.Add(sb.ToString()); outside the while to catch whatever is left in sb?

sb.Clear();
}
else
{
sb.Append(nextChar);
}

offset += 2;
}

allSubstringsWithNulls = allSubstringsWithNulls.TrimEnd('\0');

strColl.AddRange(allSubstringsWithNulls.Split('\0'));
}


Expand Down