microsoft/aspire · error · InvalidOperationException

Cannot build connection string for MongoDB replica set…

Error message

Cannot build connection string for MongoDB replica set resource '{Name}' because it does not have any members.

What it means

MongoDBReplicaSetResource builds its connection string as a seed list mongodb://host:port,host:port/?replicaSet=... from its member resources. If the replica set has no members registered, there is nothing to build the seed list from, so BuildConnectionString throws this InvalidOperationException.

Solutions

1. Register members on the replica set with WithMember (or pass the intended member count) before the app starts / before anything consumes the connection string.
2. Check your AppHost code to ensure AddMongoDBReplicaSet is not called with 0 members or members never added due to conditional logic.
3. If you only need a single server, use AddMongoDB directly instead of an empty replica set.

Example fix

// before
var rs = builder.AddMongoDBReplicaSet("mongo", 0);

// after
var rs = builder.AddMongoDBReplicaSet("mongo", 3);

if (replicaSet.Members.Count() == 0)
    throw new InvalidOperationException("Replica set must have at least one member before its connection string is used.");

When it happens

Trigger: Evaluating the replica set's ConnectionStringExpression (e.g. when a consuming project references it) after creating the replica set with zero members — e.g. AddMongoDBReplicaSet called without adding members via WithMember, or a members count of 0.

Common situations: Creating a replica set resource programmatically without adding members; a code path that removes all members before the connection string is resolved at startup; consumer projects binding to the replica set's connection string before WithMember calls complete.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• Connection string is unavailable
• The connection string of MongoDB replica set member
• Cannot remove '"))} from the existing MongoDB replica set '…
• Connection string is unavailable
• ConnectionStringAvailableEvent was published for the

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/7e68ecf005477db2. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.MongoDB/ReplicaSet/MongoDBReplicaSetResource.cs:84

    /// </summary>
    internal HashSet<string> StoppedMembers { get; } = new(StringComparers.ResourceName);

    /// <summary>
    /// Whether the replica set has been configured, meaning it can be reported as running again if its members come back.
    /// </summary>
    internal bool IsConfigured { get; set; }

    /// <summary>
    /// Gets the MongoDB server resources that are members of this replica set, in the order they were added.
    /// </summary>
    public IEnumerable<MongoDBServerResource> Members => Annotations.OfType<MongoReplicaSetMemberAnnotation>().Select(a => a.Member);

    private ReferenceExpression BuildConnectionString()
    {
        var membersList = Members.ToList();
        if (membersList is [])
        {
            throw new InvalidOperationException($"Cannot build connection string for MongoDB replica set resource '{Name}' because it does not have any members.");
        }

        var builder = new ReferenceExpressionBuilder();

        // Build the seed list `mongodb://host1:port1,host2:port2,.../?replicaSet=<name>` — see https://www.mongodb.com/docs/manual/reference/connection-string/#dns-seedlist-connection-format
        builder.AppendLiteral("mongodb://");

        if (SharedUserNameParameter is not null)
        {
            builder.Append($"{SharedUserNameParameter:uri}:{SharedPasswordParameter:uri}@");
        }
        else
        {
            builder.Append($"{MongoDBServerResource.DefaultUserName:uri}:{SharedPasswordParameter:uri}@");
        }

        for (var i = 0; i < membersList.Count; i++)
        {

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