Keywords

EF Core validation
SQL Server validation
Prevent "String or binary data would be truncated" error
Entity Framework Core interceptor
Database schema validation

DBGuard – EF Core Schema Validation Library

DBGuard is a schema-aware validation library for Entity Framework Core that prevents common SQL Server errors before data reaches the database.

It automatically validates entity data against the actual database schema and throws clear, developer-friendly validation errors instead of cryptic SQL exceptions.

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Why DBGuard?

When saving data using EF Core, SQL Server often throws errors like:

String or binary data would be truncated

This error does not tell you:

• Which table failed
• Which column caused the issue
• What the allowed size is
• What the actual value size is

DBGuard solves this problem by validating data before the database insert happens.

Example DBGuard error:

DBGuard Validation Failed

Table: Users
Column: FirstName
Allowed Length: 20
Actual Length: 45

Suggestion: Trim input or increase column length

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Features

• Schema-aware validation using actual database schema
• Automatic validation before SaveChanges
• Column length validation
• Null constraint validation
• Data type validation
• Decimal precision validation
• Numeric overflow validation
• Date validation
• EF Core SaveChanges interceptor
• Built-in logging support
• Validation diagnostics metrics
• Clear developer-friendly error messages

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Requirements

• .NET 8+
• Entity Framework Core
• SQL Server

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Installation

Install via NuGet Package Manager

Install-Package DBGuard

or using .NET CLI

dotnet add package DBGuard

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Getting Started

Follow these steps to start using DBGuard.

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Step 1 — Configure DBGuard

Add DBGuard to your service collection.

builder.Services.AddDbGuard(connectionString);

This loads the database schema and registers the validation engine.

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Step 2 — Register EF Core DbContext

builder.Services.AddDbContext<AppDbContext>((sp, options) =>
{
    var interceptor = sp.GetRequiredService<DbGuardInterceptor>();

    options.UseSqlServer(connectionString)
           .AddInterceptors(interceptor);
});

DBGuard will now automatically validate entities before saving.

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Step 3 — Example Entity

public class User
{
    public int Id { get; set; }

    public string FirstName { get; set; }

    public string Email { get; set; }
}

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Step 4 — Example Database Table

CREATE TABLE Users
(
    Id INT IDENTITY PRIMARY KEY,
    FirstName VARCHAR(20),
    Email VARCHAR(50)
)

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Step 5 — Example Insert

var user = new User
{
    FirstName = "THIS_IS_A_VERY_LONG_NAME_EXCEEDING_COLUMN_LIMIT",
    Email = "test@test.com"
};

context.Users.Add(user);

await context.SaveChangesAsync();

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Example Error Output

Instead of SQL Server throwing:

String or binary data would be truncated

DBGuard throws:

DBGuard Validation Failed

Table: Users
Column: FirstName
Allowed Length: 20
Actual Length: 45

Suggestion: Trim input or increase column length

This makes debugging significantly easier.

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Logging

DBGuard integrates with Microsoft.Extensions.Logging.

Example configuration:

builder.Services.AddLogging(config =>
{
    config.AddConsole();
});

Example log output:

warn: DBGuardInterceptor

DBGuard validation failed.
Table: Users
Column: FirstName
Error: FirstName exceeds max length 20

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Diagnostics Metrics

DBGuard tracks frequently failing columns to help diagnose issues.

Example usage:

var metrics = serviceProvider.GetRequiredService<ValidationMetrics>();

var failures = metrics.GetTopFailures();

foreach (var f in failures)
{
    Console.WriteLine($"{f.Key} -> {f.Value}");
}

Example output:

Users.FirstName -> 3 failures
Users.Email -> 1 failure

This helps identify problematic fields in large systems.

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Supported Validations

DBGuard currently supports:

• String length validation
• Null validation
• Data type validation
• Decimal precision validation
• Numeric overflow validation
• Date validation

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Project Architecture

DBGuard
│
├── DBGuard.Core
├── DBGuard.Schema
├── DBGuard.Validation
├── DBGuard.SqlServer
├── DBGuard.EFCore
│
├── samples
│   └── DBGuard.Demo
│
└── tests
    └── DBGuard.Tests

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Example Use Case

Without DBGuard:

SqlException: String or binary data would be truncated

With DBGuard:

DBGuard Validation Failed

Table: Products
Column: Description
Allowed Length: 100
Actual Length: 140

Developers immediately know which column caused the issue.

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Troubleshooting

Login failed for user

Ensure your connection string is correct and you are using either:

Windows Authentication:

Trusted_Connection=True

or SQL Authentication:

User Id=yourUser;
Password=yourPassword;

Do not use both together.

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Table not found in schema

Ensure the table exists in the database and DBGuard can access it.

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Roadmap

Future improvements:

• PostgreSQL support
• MySQL support
• SQL error analyzer
• Performance optimizations
• Monitoring dashboard
• Additional validation rules

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Contributing

Contributions are welcome.

1. Fork the repository
2. Create a feature branch
3. Submit a pull request

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Article

Read full explanation here: (https://dev.to/birupakhya_dash_b781f4db3/fixing-the-string-or-binary-data-would-be-truncated-error-in-ef-core-2dji)

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License

MIT License

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Author

Birupakhya Dash

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serious open-source library.