.NET string formatting in bicep is not supported (only rust style is)

[Gijsreyn]

Prerequisites

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• [x] Make sure you are able to repro it on the latest version
• [x] Search the existing issues.

Summary

As a user coming from the Bicep world, I would expect the following .NET notation to work (but it doesn't):

output: format('Hex formats: lowercase {0:x2}, uppercase {1:X2}, decimal {2:N0}', hexNumber, hexNumber, hexNumber)

Instead, we have to use:

output: format('Hex formats: lowercase {0:x}, uppercase {1:X}, decimal {2}', hexNumber, hexNumber, hexNumber)

For a full example, see:

targetScope = 'desiredStateConfiguration'

// Parameters for customizing the configuration
@description('Hexadecimal number for precision example')
param hexNumber int = 255

// Echo resource demonstrating precision with .NET-style formatting
resource precisionEcho 'Microsoft.DSC.Debug/Echo@2025-08-27' = {
  name: 'precisionExample'
  properties: {
    output: format('Hex formats: lowercase {0:x2}, uppercase {1:X2}, decimal {2:N0}', hexNumber, hexNumber, hexNumber)
  }
}

Versus:

targetScope = 'desiredStateConfiguration'

// Parameters for customizing the configuration
@description('Hexadecimal number for precision example')
param hexNumber int = 255

// Echo resource demonstrating precision with hexadecimal formatting
resource precisionEcho 'Microsoft.DSC.Debug/Echo@2025-08-27' = {
  name: 'precisionExample'
  properties: {
    output: format('Hex formats: lowercase {0:x}, uppercase {1:X}, decimal {2}', hexNumber, hexNumber, hexNumber)
  }
}

I wasn't sure if this was an issue or a feature request. But if it is possible, I prefer to align with the .NET style or document it somewhere.

Steps to reproduce

N/A.

Expected behavior

N/A.

Actual behavior

N/A.

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      7.5.3
PSEdition                      Core
GitCommitId                    7.5.3
OS                             Microsoft Windows 10.0.26100
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Version

dsc 3.2.0-preview.5

Visuals

No response

[SteveL-MSFT]

The challenge is that Bicep simply leverages .NET formatting. DSC has to reimplement it. We have to go case by case to implement the necessary ones that people use.