How to Make Properties Immutable After Initialization in C#

Author: Paul A. Jones Jr., MSE

In domains like legal tech, consistency is everything. Once critical information like a discovery date or a statutory deadline is captured, it should never be silently modified elsewhere in the system. Enforcing immutability in your data models ensures reliability and protects the integrity of calculations driven by complex jurisdictional rules.

This article walks through several practical approaches to making object properties immutable after initialization in C#, from traditional readonly fields to modern record types and init setters introduced in C# 9.

Why Immutability Matters (Especially in Legal Tech)

Prevents accidental mutation.
Supports thread-safety and predictability.
Aligns with functional programming best practices.
Helps you build reliable, testable domain logic.

Target Compatibility

.NET Version: .NET 6 or higher
C# Version: C# 9.0+

Three Approaches to Property Immutability in C#

Method 1: Readonly Auto-Properties (Classic Approach)

class SearchCriteria {
    public readonly int CaseTypeId;
    public readonly int JurisdictionId;
    public readonly DateTime IncidentDate;

    public SearchCriteria(int caseTypeId, int jurisdictionId, DateTime incidentDate) {
        CaseTypeId = caseTypeId;
        JurisdictionId = jurisdictionId;
        IncidentDate = incidentDate;
    }
}

Pros: Simple and effective
Cons: No object initializer support, not true properties

Method 2: Init-Only Setters (C# 9.0 and Later)

class SearchCriteria {
    public int CaseTypeId { get; init; }
    public int JurisdictionId { get; init; }
    public DateTime IncidentDate { get; init; }
}

var criteria = new SearchCriteria {
    CaseTypeId = 1,
    JurisdictionId = 2,
    IncidentDate = DateTime.Now
};

Pros: Supports object initializers, full property semantics
Cons: Potential exposure via reflection

Method 3: Use a record Type

public record SearchCriteria(
    int CaseTypeId,
    int JurisdictionId,
    DateTime IncidentDate
);

Pros: Succinct, immutable by default, value equality
Cons: Requires C# 9+, new to some teams

Choosing the Right Strategy

Method	Ideal When…
Readonly fields	You want absolute immutability with no setters
Init setters	You need immutability with flexible initialization
Records	You want full immutability and value semantics

Final Thoughts

Immutability isn’t just a programming nicety — in domains like legal tech, it’s a safeguard. Whether you’re building jurisdiction-aware logic or designing rules engines that drive your system, immutability keeps your data consistent and your logic dependable.

Choose the approach that best fits your architecture, and lean into the safety and predictability that immutable objects provide.

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