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Template Method Pattern

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using System; using System.Collections.Generic; using System.Linq; ExportResult result = new UpperReport().Run(" hello "); Console.WriteLine(result.Success + ": " + result.Text); sealed record ExportResult(bool Success, string Text); abstract class ReportTemplate { public ExportResult Run(string input) { string value = input.Trim(); if (value.Length == 0) return new ExportResult(false, "EMPTY_INPUT"); return new ExportResult(true, "REPORT|" + Format(value)); } protected abstract string Format(string value); } sealed class UpperReport : ReportTemplate { protected override string Format(string value) => value.ToUpperInvariant(); } sealed class JsonReport : ReportTemplate { protected override string Format(string value) => System.Text.Json.JsonSerializer.Serialize(value); }

using System; using System.Collections.Generic; using System.Linq; ExportResult result = new UpperReport().Run(" hello "); Console.WriteLine(result.Success + ": " + result.Text); sealed record ExportResult(bool Success, string Text); // Keep the shared sequence and validation rules in the base class. abstract class ReportTemplate { public ExportResult Run(string input) { string value = input.Trim(); if (value.Length == 0) return new ExportResult(false, "EMPTY_INPUT"); return new ExportResult(true, "REPORT|" + Format(value)); } protected abstract string Format(string value); } // Subclasses change this formatting step rather than the whole workflow. sealed class UpperReport : ReportTemplate { protected override string Format(string value) => value.ToUpperInvariant(); } sealed class JsonReport : ReportTemplate { protected override string Format(string value) => System.Text.Json.JsonSerializer.Serialize(value); }

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using System; using System.Collections.Generic; using System.Linq; ReportTemplate report = new JsonReport(); foreach (string input in new[] { " hello ", " " }) { ExportResult result = report.Run(input); Console.WriteLine(result.Success + ": " + result.Text); } sealed record ExportResult(bool Success, string Text); abstract class ReportTemplate { public ExportResult Run(string input) { string value = input.Trim(); if (value.Length == 0) return new ExportResult(false, "EMPTY_INPUT"); return new ExportResult(true, "REPORT|" + Format(value)); } protected abstract string Format(string value); } sealed class UpperReport : ReportTemplate { protected override string Format(string value) => value.ToUpperInvariant(); } sealed class JsonReport : ReportTemplate { protected override string Format(string value) => System.Text.Json.JsonSerializer.Serialize(value); }

using System; using System.Collections.Generic; using System.Linq; ReportTemplate report = new JsonReport(); foreach (string input in new[] { " hello ", " " }) { ExportResult result = report.Run(input); Console.WriteLine(result.Success + ": " + result.Text); } sealed record ExportResult(bool Success, string Text); // Keep the shared sequence and validation rules in the base class. abstract class ReportTemplate { public ExportResult Run(string input) { string value = input.Trim(); if (value.Length == 0) return new ExportResult(false, "EMPTY_INPUT"); return new ExportResult(true, "REPORT|" + Format(value)); } protected abstract string Format(string value); } // Subclasses change this formatting step rather than the whole workflow. sealed class UpperReport : ReportTemplate { protected override string Format(string value) => value.ToUpperInvariant(); } sealed class JsonReport : ReportTemplate { protected override string Format(string value) => System.Text.Json.JsonSerializer.Serialize(value); }

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TypeScript example The base class owns trimming, empty-input validation and the prefix; format() holds the representation-specific step. Formats can change without copying and potentially omitting common checks. type ExportResult = { success: true; text: string } | { success: false; code: "EMPTY_INPUT" }; // Keep the shared sequence and validation rules in the base class. abstract class ReportTemplate { run(value: string): ExportResult { const clean = value.trim(); if (clean.length === 0) return { success: false, code: "EMPTY_INPUT" }; return { success: true, text: "REPORT|" + this.format(clean) }; } protected abstract format(value: string): string; } // Subclasses change this formatting step rather than the whole workflow. class UpperReport extends ReportTemplate { protected override format(value: string): string { return value.toUpperCase(); } } class JsonReport extends ReportTemplate { protected override format(value: string): string { return JSON.stringify({ value }); } } for (const report of [new UpperReport(), new JsonReport()]) { const result = report.run(" hello "); console.log(result.success ? result.text : result.code); } const empty = new UpperReport().run(" "); console.log(empty.success ? empty.text : empty.code); REPORT|HELLO REPORT|{"value":"hello"} EMPTY_INPUT TypeScript has no final restriction equivalent to the non-virtual C# method; these subclasses implement format() without overriding run(). Empty input returns a failure before formatting is called.

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Each example is a separate .NET 10 console program.

Each C# example is a separate .NET 10 console program. Compile the TypeScript examples for ES2022 or later and run the emitted JavaScript with Node.js.