Replace Presentation Fonts
Replace an explicit font reference across a presentation and inspect the saved result.
Input and prerequisites
Use .NET SDK 8.0.427 and Aspose.Slides for .NET 26.10.0 with the supplied pinned project. Set ASPOSE_LICENSE_PATH to your own valid license file; no license file is distributed. This complete fixture stops clearly when no license has been applied.
The code downloads pinned Noto fonts from their official repositories and verifies SHA-256 before loading them from an application folder. Internet access is required on the first run; the fonts are not installed system-wide or included in this project ZIP. See the Noto font license and Noto CJK license. Both are SIL Open Font License 1.1. Preserve their notices if you redistribute the fonts.
Download the complete project, including code, project and package lock. It was executed on Windows.
Example
The fixture has explicit Arial text and a paragraph-default Arial font. ReplaceFont changes the font references to the application-loaded Noto Sans family. The program reopens the file and inspects both kinds of formatting before rendering it.
using System;
using System.Collections.Generic;
using System.Drawing;
using System.IO;
using System.IO.Compression;
using System.Linq;
using System.Net.Http;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Text.Json;
using System.Text.RegularExpressions;
using Aspose.Slides;
using Aspose.Slides.Export;
using Aspose.Slides.Warnings;
internal static class Program
{
private static void Main()
{
ApplyLicense();
PrepareFonts(false);
try
{
using (var source = new Presentation())
{
source.SlideSize.SetSize(640, 360, SlideSizeScaleType.DoNotScale);
var explicitText = Label(source.Slides[0], "Explicit font");
explicitText.Name = "explicit";
explicitText.TextFrame.Paragraphs[0].Portions[0].PortionFormat.LatinFont = new FontData("Arial");
var defaultText = Label(source.Slides[0], "Paragraph default", 175);
defaultText.Name = "default";
defaultText.TextFrame.Paragraphs[0].ParagraphFormat.DefaultPortionFormat.LatinFont = new FontData("Arial");
source.Save("input.pptx", SaveFormat.Pptx);
}
string inputHash = Hash("input.pptx");
using (var editable = new Presentation("input.pptx"))
{
editable.FontsManager.ReplaceFont(new FontData("Arial"), new FontData("Noto Sans"));
editable.Save("output.pptx", SaveFormat.Pptx);
}
using (var result = new Presentation("output.pptx"))
{
var a = result.Slides[0].Shapes.OfType<IAutoShape>().Single(s => s.Name == "explicit");
var b = result.Slides[0].Shapes.OfType<IAutoShape>().Single(s => s.Name == "default");
Require(a.TextFrame.Paragraphs[0].Portions[0].PortionFormat.LatinFont.FontName == "Noto Sans", "Explicit reference was not replaced.");
Require(b.TextFrame.Paragraphs[0].ParagraphFormat.DefaultPortionFormat.LatinFont.FontName == "Noto Sans", "Paragraph default was not replaced.");
Require(a.TextFrame.Text == "Explicit font" && b.TextFrame.Text == "Paragraph default", "Text changed.");
Render(result.Slides[0], "preview.png");
}
Require(Hash("input.pptx") == inputHash, "Original changed.");
Report(new { explicit_replaced = true, paragraph_default_replaced = true, saved_text_retained = true, original_unchanged = true });
}
finally { FontsLoader.ClearCache(); }
}
private static void PrepareFonts(bool includeOtherScripts)
{
Directory.CreateDirectory("fonts");
using var client = new HttpClient { Timeout = TimeSpan.FromMinutes(2) };
var fonts = new (string Name, string Url, string Hash)[]
{
("NotoSans-Regular.ttf", "https://raw.githubusercontent.com/notofonts/noto-fonts/ffebf8c1ee449e544955a7e813c54f9b73848eac/hinted/ttf/NotoSans/NotoSans-Regular.ttf", "b85c38ecea8a7cfb39c24e395a4007474fa5a4fc864f6ee33309eb4948d232d5"),
("NotoNaskhArabic-Regular.ttf", "https://raw.githubusercontent.com/notofonts/noto-fonts/ffebf8c1ee449e544955a7e813c54f9b73848eac/hinted/ttf/NotoNaskhArabic/NotoNaskhArabic-Regular.ttf", "2f4b88e6ee50fa82c617e2d1d4ba18281cb1c6cd71c3af3ec64970c23995db4b"),
("NotoSansCJKsc-Regular.otf", "https://raw.githubusercontent.com/notofonts/noto-cjk/f8d157532fbfaeda587e826d4cd5b21a49186f7c/Sans/OTF/SimplifiedChinese/NotoSansCJKsc-Regular.otf", "2c76254f6fc379fddfce0a7e84fb5385bb135d3e399294f6eeb6680d0365b74b"),
};
foreach (var font in includeOtherScripts ? fonts : fonts.Take(1))
{
string path = Path.Combine("fonts", font.Name);
if (!File.Exists(path) || Hash(path) != font.Hash)
{
byte[] bytes = client.GetByteArrayAsync(font.Url).GetAwaiter().GetResult();
Require(Convert.ToHexString(SHA256.HashData(bytes)).ToLowerInvariant() == font.Hash, "Downloaded font hash differs: " + font.Name);
File.WriteAllBytes(path, bytes);
}
}
FontsLoader.ClearCache();
FontsLoader.LoadExternalFonts(new[] { Path.GetFullPath("fonts") });
}
private static void ApplyLicense()
{
var license = new Aspose.Slides.License();
string? path = Environment.GetEnvironmentVariable("ASPOSE_LICENSE_PATH");
if (!string.IsNullOrWhiteSpace(path)) license.SetLicense(path);
if (!license.IsLicensed()) throw new InvalidOperationException("Set ASPOSE_LICENSE_PATH to a valid license file.");
}
private static void Require(bool condition, string message)
{
if (!condition) throw new InvalidOperationException(message);
}
private static IAutoShape Label(IBaseSlide slide, string text, float y = 50)
{
var shape = slide.Shapes.AddAutoShape(ShapeType.Rectangle, 40, y, 560, 90);
shape.TextFrame.Text = text;
shape.TextFrame.Paragraphs[0].Portions[0].PortionFormat.FontHeight = 24;
return shape;
}
private static string Hash(string path) => Convert.ToHexString(SHA256.HashData(File.ReadAllBytes(path))).ToLowerInvariant();
private static void Report(object checks)
{
string json = JsonSerializer.Serialize(new { checks }, new JsonSerializerOptions { WriteIndented = true });
File.WriteAllText("result.json", json);
Console.WriteLine(json);
}
private static void Render(ISlide slide, string output)
{
using var image = slide.GetImage(new Size(640, 360));
image.Save(output, Aspose.Slides.ImageFormat.Png);
}
}
Run and inspect the result
Extract the download into an empty directory and run:
dotnet restore Example.csproj --locked-mode
dotnet run --project Example.csproj --configuration Release --no-restore
The code writes “input.pptx”, “output.pptx”, “preview.png”, plus “result.json”. Checks stop execution on an unexpected result; inspect visual outputs as well as the recorded data.
Practical limits
Fonts can also come from theme major/minor settings, defaults and script-specific formatting. Inspect effective formatting and adjust a theme deliberately when that is the intended design change. Replacing a font does not guarantee identical character widths, wrapping or appearance.
Read the detailed guide for the full API and platform requirements.